interest_calculator.zip
I only code in Visual Basic so can offer a solution to your question that way. There are free VB to C# converters on the web and they are easy to use. Here is the solution to your problem in VB:
"YourTextBox".Text = String.Format("{0:n2}", "YourVariable"
Substitute "YourTextBox" and "YourVariable" with the names that you have used but do not include the quotation marks.
HTH