#### Answer

The negative exponent rule: $a^{−n}=\frac{1}{a^{n}}$ and $\frac{1}{a^{-n}} = a^{n}$ says that a term with a negative exponent becomes the same term with a positive exponent if it is moved across the fraction bar—from numerator to denominator or vice versa.
For instance, consider $\frac{x^{-2}}{y^{-3}}$. In simplifying this, move $x^{-2}$ to the denominator and change the power to $+2$. Do the same for $y^{-3}$, move it to the numerator and change the exponent to $+3$. Thus,
$$\frac{x^{-2}}{y^{-3}} =\frac{y^{3}}{x^{2}}$$