If you want to see the total of all the found records then you can setup a Summary field that Total your po_amount field, so when creating a field specify the Type: as Summary, and create a second one for po_cancel and then a Calculation that Subtract one from the other.
Sum_amount - Sum_cancel
If you are looking to show the total for a specific grouping, it will depend on how the two tables are linked and which one you are viewing the data from, but you can use the the Calculation Function Sum ( RELATED TABLE:: po_amout ) to total all the po_amount values in the related records, and again do the same for po_cancel if you want to desplay them seperate or subtract one from the other in the one calculation.
Sum ( RELATED TABLE:: po_amout ) - Sum ( RELATED TABLE:: po_cancel )
I hope this makes sense and helps.
Thanks, I got it.