Function question regarding margins and printed areas?

The top and bottom margins of a poster are each 6cm and the side margins are each 4cm. The total area of the printed matter on the poster must be 400 sq cm. If x is the length of the base of the printed matter, express the area of the printed matter as a function of x. I made A the total length and B the total width so that would give me something like: 400= (A-8) (B-12) but then (B-12) would equate to x and i'm not really sure how to go on from there