Answer
$$y'=-3^{-x}\ln3$$
Work Step by Step
$$y=3^{-x}$$
Recall Theorem 5: $$\frac{d}{dx}a^u=a^u\ln a\frac{du}{dx}$$
Apply here to find $y'$: $$y'=(3^{-x})'=3^{-x}\ln 3(-x)'$$
$$y'=-3^{-x}\ln3$$
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