You can make a negative exponent positive by switching its numerator with its denominator and vice versa.
Examples:
[tex]x^{-2} = \frac{1}{x^2}[/tex]
[tex]3x^{-2} = \frac{3}{x^2}[/tex]
[tex](3x)^{-2} = \frac{1}{(3x)^2}[/tex]
[tex]\frac{1}{x^{-2}} = x^2[/tex]
[tex]\frac{1}{3x^{-2}} = \frac{x^2}{3}[/tex]
[tex]\frac{1}{(3x)^{-2}} = \frac{(3x)^2}{1} = (3x)^2[/tex]
