(a) If $V$ is the volume of a cube with edge length $x$ and the cube expands as time passes, find $d V / d t$ in terms of $d x / d t$
(b) If the length of the edge of a cube is increasing at a rate of $4 \mathrm{~cm} / \mathrm{s}$, how fast is the volume of the cube increasing when the edge length is $15 \mathrm{~cm} ?$