Solve the Problem $V ( x ) = 5 x ^ { 2 }$

Solve the problem.
-The volume of a rectangular box with square base and a height of 5 feet is $V ( x ) = 5 x ^ { 2 }$ , where $x$ is the length of a side of the base. Find the instantaneous rate of change of volume with respect to $x$ when $x = 3$ feet.

A) $45 \mathrm { ft } ^ { 3 } / \mathrm { ft }$
B) $30 \mathrm { ft } ^ { 3 } / \mathrm { ft }$
C) $6 \mathrm { ft } ^ { 3 } / \mathrm { ft }$
D) $15 \mathrm { ft } ^ { 3 } / \mathrm { ft }$