I need help with this

Question

A square sheet is cardboard has length x on each side. Let 15 be the side of each 4 squares cut from each corner of the sheet. The sides are then folded to form an open box. What is the surface area of the outside of the box in terms of x?

Answer 1

2 sides are (15-2x)x, the other 2 sides are the same for a total 60x-8x^2. The bottom is (15-2x)^2 = 225 -60x + 4x^2

add them up

surface area = 225-4x^2