[tex]\bf \left.\qquad \qquad \right.\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}}}}\\\\
-------------------------------\\\\[/tex]
[tex]\bf \cfrac{7^{16}}{7^{12}}=\cfrac{7^{-18}}{x}\implies x\cdot 7^{16}=7^{12}\cdot 7^{-18}\implies x\cdot 7^{16}=7^{12-18}
\\\\\\
x\cdot 7^{16}=7^{-6}\implies x=\cfrac{7^{-6}}{7^{16}}\implies x=\cfrac{7^{-6}\cdot 7^{-16}}{1}\implies x=7^{-6-16}
\\\\\\
\boxed{x=7^{-22}}\implies x=\cfrac{1}{7^{22}}[/tex]
