Math Help - Maximize Volume of a Box

A 60 by 60 square piece of cardboard is to be made into a box by cutting out equal square corners from each side of the square. What size corners should be cut out so that the volume of the box is maximized?

Length of each corner : ________

volume of box = (x)(60-2x)^2
get dv/dx and let it equal zero.
Let x = 10