how to apply a formula across multiple columns?

Hi,

I barely use Excel but a client has given me a spreadsheet to work with. I have two columns and I want to run a function on the two columns and save the result in a third. I know how to do it on a per row basis but not across all rows.

I need to multiply the value in the first column by itself and then divide it by the value in the second column.

e.g (col A x col A) / col B

I would appreciate any help