Let's say you have two fields, Price and Tax both defined as currency.
Place those in columns 1 and 2 of the query design panel. In the 3rd column
put in an Expression rather than a field name.
TotalPrice: Price + Tax
If the Price is $10.00 and the tax is $3.20, the expression Total will yeild
$13.20.
SQL will look like :
SELECT tblNetprice.Price, tblNetprice.Tax, [Price]+[Tax] AS TotalPrice
FROM tblNetprice;