Sodium chlorate crystals are easy to grow in the shape of cubes by allowing a solution of water and sodium chlorate to evaporate slowly. If $V$ is the volume of such a cube with side length $x$, calculate the derivative when $x = 4 \text{ mm}$.
$V'(4) = \cdots \text{ mm}^3/\text{mm}$
I thought that:
$V'(4) = 64 \text{ mm}^3/\text{mm}$
Because volume of a cube $=$ $4 \times 4 \times 4$