Maximum Volume

From a rectangular sheet of metal 120mm by 75mm, equal squares of side x are cut from all 4 corners, the remaining sheet is folded into an open box.
A, draw a simple diagram of the rectangular sheet of metal and show the dimensions including the squares of side x
B, show that the volume of the box is given by; V=9000x-390x^2+4x^3
C, find the value of x such that the volume is a maximum