How to calculate HTML table columns (using javascript or jquery) with a user defined equation

Well, the way you are doing is very fragile, the user would input a single different character in the input and then all calcuation can fail.

But besides that, there's a way of doing that keeping the way you want (I don't recommend), but let's go to the explanation:

1. Add classes to your HTML table, in a way that you can identify each row and column.
2. Create a function that get the values from each row (using a loop) and another function that you can call to calculate those values.
3. Inside the function that calculates is where comes the solution and the problem at the same time. For now, it works only with 3 columns and considering that the user knows that only c1, c2 or c3 are valid column names.
4. With the equation value, you replace the column names with the column values. Then you check if there's some other character remaining, if yes, then the equation is wrong and a message or an error should be displayed.
5. If no other characters remain and just numbers and operation symbols are in the equation, then we use the BAD IDEA eval() method, which evaluates the equation as it is.
6. I added the eval function inside a try...catch block to let it a little more secure (Please, read about why eval is not a good solution here: LINK)
7. Well, if the equation was typed correct and no malicious code was introduced by eval, then you will have the final value to return.

Below code as example:

let btn_calc = $("#btn_calc");
let equation = $("#equation");


btn_calc.on("click", calculate);

function calculate(){
  let rows = $(".values");
  for (var i = 0; i < rows.length; i++){
    let singleRow = $(rows[i]);
    let grade = singleRow.find(".final-grade");
    let n1 = singleRow.find(".c1").text(); 
    let n2 = singleRow.find(".c2").text();
    let n3 = singleRow.find(".c3").text();
    let total = returnValue(n1,n2,n3);
    if (total != false){
      grade.text(total)    
    } else {
      return;
    }
  }
}

function returnValue(n1,n2,n3){
  let calc = equation.val().trim();
  let result = 0;
  debugger;
  if (calc != null && calc != ""){
     calc = calc.replace(/c1/g,n1).replace(/c2/g,n2).replace(/c3/g,n3)

     var regx = new RegExp("[a-z]", "gi");
     debugger;
     if (calc.match(regx)){
      console.log(calc);
      alert("Your equation is invalid!");
      return false;
     }

     try{
      result = eval(calc);
      result.toFixed(2);
     } catch (ex){
      alert("Error in your equation! " + ex);
     }
  }
  return result;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div>
  <input type="text" id="equation"/> (Valid Column names: c1, c2, c3)
  <br>
  <input type="button" value="Calculate" id="btn_calc"/>
</div>
<table>
  <tr>
        <th>Student ID</th>
        <th >Homework 1</th>
        <th >Homework 2</th>
        <th>Exam points</th>
        <th>Final grade</th>
  </tr>
  <tr class="values">
        <td>1</td>                              
        <td class="c1">8.75</td>
        <td class="c2">7.60</td>
        <td class="c3">55.50</td>
        <td class="final-grade">-</td>
    </tr>
    <tr class="values">
        <td>2</td>
        <td class="c1">9.00</td>
        <td class="c2">4.50</td>
        <td class="c3">63.00</td>
        <td class="final-grade">-</td>
    </tr>
    <tr class="values">
        <td>3</td>
        <td class="c1">8.75</td>
        <td class="c2">7.60</td>
        <td class="c3">55.50</td>
        <td class="final-grade">-</td>
</tr>

You're lucky that I had some free time to code this for you... I should send the bill hahaha

As long as your html structure stays simple and you are okay with using eval (which is actually not recommended), defining the following JavaScript code and invoking it will do the trick for you. You will need to ensure that c1...c3 have valid values in them.

    function calcTotals() {
        const c1 = 'parseFloat(fg.previousElementSibling.previousElementSibling.previousElementSibling.textContent)'
        const c2 = 'parseFloat(fg.previousElementSibling.previousElementSibling.textContent)'
        const c3 = 'parseFloat(fg.previousElementSibling.textContent)'
        const equation = document.getElementById('equation').value
        try {
            const fg = document.querySelector('td.final-grade')
            eval(equation.replace('c1',c1)
                         .replace('c2',c2)
                         .replace('c3',c3))
        } catch(e) {
            alert('Please enter a valid equation')
        }
        document.querySelectorAll('td.final-grade').forEach(fg => {
            fg.textContent = eval(equation.replace('c1',c1)
                                          .replace('c2',c2)
                                          .replace('c3',c3))
        })
    }