If 1900 square centimeters of material is available to make a box with a square base and an open top, find the largest possible volume of the box.

so i tried doing this

v=x^2y

x^2+4xy=1900

y=(1900-x^2)/4x

=475x-x/4

V=x^2(475x-x/4)

taking the derivative and setting it to 0 i get

12.58 but thats not right

can anyone see what i did wrong