to the risk of sounding redundant.
[tex] \bf ~~~~~~~~~~~~\textit{negative exponents}
\\\\
a^{-n} \implies \cfrac{1}{a^n}
\qquad \qquad
\cfrac{1}{a^n}\implies a^{-n}
\qquad \qquad
a^n\implies \cfrac{1}{a^{-n}}
\\\\
-------------------------------\\\\
f(-3)=2^{-3}\implies f(-3)=\cfrac{1}{2^3}\implies f(-3)=\cfrac{1}{8} [/tex]