Any help is greatly appreciated!

I have two tables like so:

Tb1:
AutoNum Agreement SubAgreement Value ValueLeft
1 Agree1 Sub1 $10
2 Agree1 Sub2 $15 X
3 Agree2 Sub1 $8
4 Agree2 Sub2 $17

Tb2:
AutuNum InvoiceNum Agreement SubAgreement AmountBilled
1 1234 Agree1 Sub2 $4
2 5678 Agree1 Sub2 $3


My question is, how do I get X in Tb1.

The value of X should be "X = $15 - $4 - $3"

[ValueLeft] needs to look at Tb2 where [Agreement] and [SubAgreement] equals [Agreement] and [SubAgreement] in Tb1 and subtract all [AmountBilled] values from [Value].

These tables are displayed in a form.