50 POINTS - PLEASE HELP!!! A supermarket display consists of boxes of cereal. The bottom row has 40 boxes. Each row has three fewer boxes than the row below it. The display has seven rows.
a. Write and use a function to determine how many boxes are in the top row (show your work)
b. Use the appropriate formula to determine the number of boxes in the entire display (show your work)

I think I have the 1st part down:
40+(7-1)x-3
40+(6)x-3
23+(-18)
=5 Number of boxes on the Top Row