How to use extension methods embedded with HTML Helper in razor view

I am learning ASP.Net MVC 5 , and I am stuck at usage of extention method. So I created an extension method and now I want to use it in my Razor view. But it's throwing error InvalidOprationException Error.

Model

public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    [Display(Name = "Amount Owed")]
    public decimal Amount { get; set; }
}

View

@model IEnumerable<WebApplication3.Models.Student>
@using WebApplication3.Extension
<table class="table" id="studentstable" style="border: 1px solid black; background-color: silver">
<tr>
    <th>
        @Html.DisplayNameFor(model => model.Name)
    </th>
    <th>
        @Html.DisplayNameFor(model => model.Amount)
    </th>
    <th>
        @Html.DisplayNameFor(model => model.Amount)
    </th>
</tr>

@foreach (var item in Model)
{
    <tr>
        <td>
            @Html.DisplayFor(modelItem => item.Name)
        </td>
        <td>
            @item.Amount.ConvertToDollar() @* this works fine *@
        </td>
        <td>
            @Html.DisplayFor(modelItem => item.Amount.ConvertToDollar())  @*I want somwething like this so that I do not lose the HTML HELPERs*@
        </td>
    </tr>
}
</table>

Extension Method

namespace WebApplication3.Extension
{
    public static class Helper
    {
        public static string ConvertToDollar(this decimal amount)
        {
            return String.Format("{0:C}", amount);
        }
    }
}

Error Line:

@Html.DisplayFor(modelItem => item.Amount.ConvertToDollar()). I can use simply @item.Amount.ConvertToDollar() But I want to embed it in HTMLHELPER. Please guide me. Is this even possible?