[tex]~\hspace{7em}\textit{negative exponents} \\\\ a^{-n} \implies \cfrac{1}{a^n} ~\hspace{4.5em} a^n\implies \cfrac{1}{a^{-n}} \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ 3^x=\cfrac{1}{9}\implies 3^x=\cfrac{1}{3^2}\implies 3^x=3^{-2}\implies x = -2[/tex]
