I assume this is a simplification, one may note you never really asked anything, but let's assume simplification
well, when all you have is factors atop and bottom, you can simply move them up or down, just recall, their sign must change though, is all there's to it
[tex]\bf a^{-{ n}} \implies \cfrac{1}{a^{ n}}\qquad \qquad
\cfrac{1}{a^{ n}}\implies a^{-{ n}}\\\\
-------------------------------\\\\
\cfrac{3^{-9}}{3^{-12}}\implies 3^{-9}\cdot 3^{12}\implies 3^{-9+12}\implies 3^3\implies 27[/tex]
