calling getpaymentamount into my gui.

Question

Well it is apparent i am new to c# cause i am having a problem calling a GetPaymentAmount into my GUI.

This is my interface:
namespace Invoice
{
    public interface IPaytable
    {
        decimal GetPaymentAmount();
    }
}
This is my method
public decimal GetPaymentAmount()
        {
           
               return priceper * quantity;
             
            }
and i am completely lost as how to make it to where i can display the information from the getpaymentamount into my Results textbox as such:
restxtbx.Text += "Total for this transaction is :$" + WHAT GOES HERE?  ;