newbie question: multiple and deduct data retrieved from subform

[cuongvt]

Hi,
I have a main form named product with 3 subform: Initialstock, take in, take out
I created a text box named "remained" in mainform and want to sum the Initialstock with take in, and then deduct the take out by this:
=InventorySubform!initialstock+SubDelivery!shiped-[Stockout Subform]!out

but it not works. could you tell me in detail the way to solve it. thanks