[tex]\bf ~\hspace{7em}\textit{negative exponents} \\\\ a^{-n} \implies \cfrac{1}{a^n} ~\hspace{4.5em} a^n\implies \cfrac{1}{a^{-n}} ~\hspace{4.5em} \cfrac{a^n}{a^m}\implies a^na^{-m}\implies a^{n-m} \\\\[-0.35em] \rule{34em}{0.25pt}[/tex]
[tex]\bf f(x)=\cfrac{20}{4+3e^{-0.2x}}\implies f(3)=\cfrac{20}{4+3e^{-0.2(\stackrel{\downarrow }{3})}}\implies f(3)=\cfrac{20}{4+3e^{-0.6}} \\\\\\ f(3)=\cfrac{20}{4+3\frac{1}{e^{0.6}}}\implies f(3)=\cfrac{20}{4+3\frac{1}{e^{\frac{3}{5}}}}\implies f(3)=\cfrac{20}{4+3\frac{1}{\sqrt[5]{e^3}}} \\\\\\ f(3)=\cfrac{20}{4+\frac{3}{\sqrt[5]{e^3}}}\implies f(3)\approx\cfrac{20}{4+5.6464}\implies f(3)\approx 3.5421[/tex]
you could also just use plug in 0.6 as the exponent for "e", Euler's constant, in your calculator, and that works the same.
