Help with calculus please

An open box is to be made from a twenty-four-inch by twenty-four-inch square piece of material by cutting equal squares from the corners and turning up the sides, (where y = 24 − 2x). Find the volume of the largest box that can be made.
V = in3

V = L * W * H

L = W
L = 24 - 2x
H = x

V = (24 - 2x) * (24 - 2x) * x
V = 2 * 2 * (12 - x)^2 * x
V = 4 * (12 - x)^2 * x
dV/dx = 4 * (x * d/dx (12 - x)^2 + (12 - x)^2 * d/dx x)
dV/dx = 4 * (x * 2 * (-1) * (12 - x) + (12 - x)^2 * 1)
dV/dx = 4 * (12 - x) * (12 - x - 2x)
dV/dx = 4 * (12 - x) * (12 - 3x)
dV/dx = 4 * 3 * (12 - x) * (4 - x)
dV/dx = 12 * (12 - x) * (4 - x)

dV/dx = 0
0 = 12 * (12 - x) * (4 - x)

12 - x = 0
x = 12

4 - x = 0
x = 4

V = (24 - 2x)^2 * x
V = 4 * (24 - 8)^2
V = 4 * 16^2
V = 4 * 256
V = 1024