Calculating Line-Item Cost in Form (1 Viewer)

I am having trouble calculating a field in a table and on a form.
I have the following Tables
CUSTOMER: Customer No, Name, Address, City, State, Zip, Discount, Phone, Fax, Contact Person.
ORDER: Customer No, Order No, Order-Date, Delivery-Date.
ORDER LINE: Order No, Product No, Quantity Ordered,
line-item-cost
PRODUCT: Product No, Description, Price, Product Image.

I need to calculate the line-item cost and need it to update every time the user enters a new Quantity ordered on the ORDER LINE form.
I know its (Quantity Ordered*Price)-(Price*Discount) and I did this is a query and it worked but I need it to update in the form and update in the table not the query. I have tried everything (Macros, Queries, and Formulas) and I just can not get it to work.
Does anyone know how to do this?
Thanks

Have a look at my sample posted Here