What function do i use for this question

In C4 create calculation to add the Increment (the amount our forecast is increasing by) in B3 to the volume in B4. (Note we want the ability to change the values in B3 and B3 so do NOT just add 25, use a relative reference or a range name). Drag the formula across to G4.

This is for ACCG2000 macquarie university assessment for excel