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.????

thats very messy

Dim a, b, c, d as double
a = Me.[Quantity_Ordered]
b = Me.[Price]
c = Me.[Discount]
d = ((a*b) - (b*c))
Me.[Line_Item_Cost] = d

tidier. not sure where the problem is though...can you post the database to look at?

You should not be storing this value. You can calculate this in a query (your form should be based on queries, not tables, for this and other reasons) or in your form. VBA is not needed.

What was wrong with this link?

I gave you an answer, which you did not acknowledge as unhelpful, so I assumed it solved your problem.

You have reposted here and created another thread, not only that but you have double posted on another forum.

Problem With The Query

I tried to do this from the example that you gave me, but I can’t get it to work because my information comes from 4 different tables and I just could not get them to link in the query? Is there anything I can do about that? Sorry I forgot to put down that it was helpful, it was very helpful and I thank you all.
Thanks
dBlaszak