A rectangular sheet of cardboard, 50 cm by 40 cm, is to have square corners cut of so it can be folden into a rectangular tray.

Find the maximum volume possible for the tray.

Here is what I got so far.

Volume of Tray: L x W x H

Volume of Tray with variable: L=(50 - 2x) x W(40 - 2x) x H=x

Where x is equal to the cut out squares.

(50 - 2x)(40 - 2x)x

(2000 - 100x - 80x +4x^2)x

2000x - 100x^2 - 80x^2 + 4x^3

F'(x) = 2000 - 40x + 12x^2

4(500 - 40x + 3x^2)

Ahaha, I'm not sure what is going wrong...