Simple Calculation Help

Well, I hope this is simple, but for some strange reason, very difficult for me.

I'm having issues setting up a calculation between 2 tables. for example:

Table1 ID Quantity


Table2 TransID Used


The "Quantity" from Table1 is a set number but the "Used" will change. all I want to do is have a Form and Report that will subtract the "Used" from Table2 from "Quantity" from Table1.

I've tried: =Table1.Quantity - Table2.Used
=Quantity - Used

what am I doing wrong? all my attempts have failed, even though I have setup a relationship between the 2 tables.

any idea?

thx