Help with a problem

No.11152219 ViewReplyOriginalReport
Translation: Determine the coloured rectangle's largest possible area by using derivative.

I've tried it by setting the derivative y' = 6 - 2x to 0 so that x = 3, and then setting that into the function to get 9 and thus P is on the coordinates (3;9) and miltiplying them to get 27 area units, but the answer is supposed to be 32, with P being on (4;8).

How would you solve this problem?