How can I display subtraction result in Crystal Report 9.0?

Dear Sir

I use VB 6.0, Crystal report 9 and Microsoft Access as database program. In my database, I have two tables as tblpurchaseqty & tblsupplyqty. Common field is item and I have made relation with the tables based on item field.

I have made two query files (qrypurchaseqty & qrysupplyqty) which has summed all quantity based on item from two files (i.e. from tblpurchaseqty & tblsupplyqty). Now in qrypurchaseqty file there are two field- item & sumqrypurchaseqty and in qrysupplyqty file there are also two field- item & sumqrysupply where item wise total quantity is displaying.

Now I want to subtract sumqrysupplyqty from sumqrypurchaseqty and want to display result in crystal report. In form part of crystal report I have put sumqrysupplyqty & sumqrypurchaseqty both and have written in RecordSelection part (sumqrysupplyqty-sumqrypurchaseqt), but result doesn’t display.

Please advise.

Regards
Pervez