Problem: if 1200cm^2 of material is available to make a box with a square base and an open top, find the largest possible volume of the box

Heres what I did

area = 1200cm^2

total area = area of the 5 sides of the box(5 since it says an open top)

area of one side = xy

total area = 1200=5xy

x=240/y

volume =xy^2

v=240y(y^2)

V=240y

V'=240....

which doesn't work because I think to find the maximum, you have to solve where the derivative is 0

is there another strategy or did I make some basic mistake