Numbers 09 Formula Question

[vandrv]

I am new to Numbers and am having a terrible time figuring out how to add a formula to a spreadsheet I created. I am trying to multiply two columns one of which is quantity and the other is price. I am then trying to get the result of these two columns to multiply by a tax rate, and then add the two results. I can create the formula and have the numbers come out right in each individual cell but I know there must be a simple way to have the formula apply to a whole column, but I am having no luck figuring out how. Thanks for any help I can get.