A box has square base and the sum of one side of the base and height of the box is 12. If the length of one side of box is $x$ cm:
a. Express volume $V$ of the box in terms of $x$.
b. Find

$$\frac{d}{dx}(V)$$

and

$$\frac{d^{2}}{dx^{2}}(V)\text{.}$$

c. Find the dimensions of the box for which the volume of the box is maximum. Hence find maximum volume of the box.