Algebra 2 Common Core

$$\frac{1}{mg^{3}}$$
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: $$(mg^{3})^{-1}=\frac{1}{(mg^{3})^{1}}=\frac{1}{mg^{3}}.$$