#### Answer

$$\frac{1}{mg^{3}}$$

#### Work Step by Step

An expression raised to a negative power is equal to the reciprocal of that expression raised to the same positive power. An expression raised to the first power is just that expression:
$$(mg^{3})^{-1}=\frac{1}{(mg^{3})^{1}}=\frac{1}{mg^{3}}.$$