Auto Updating Total Order Cost After Quantity Is Changed

Ok so here is my problem:
I have a form that is realted to four differnt tables with the names: CUSTOMER, ORDER, PRODUCT AND ORDER LINE. With-in the table Order-Line there is a entity called, line_item_cost whcih is the total amount of the order with respect to Quanity_Ordered which comes from the ORDER LINE table and Price which comes from the PRODUCT table and Discount (%) which comes from the CUSTOMER table. When the users goes to change the Quantity_Ordered I need the form to automaticly update the line_item_cost which would calculate [Price]*[Quantity_Ordred]-[Price]*[Discount] . I tried to build a code in VBA to do this, Here is the Code:
Dim a, b, c, d
a = Me.[Quantity_Ordered]
b = Me.[Price]
c = Me.[Discount]
d = a * b - b* c
Me.[Line_Item_Cost] = d

VBA Complies the program but it does not show up in my form.
I think I might be updating it at the wrong point.
Any Suggestions.????