Answer
$$-\frac{1}{\ln 7}7^{-x} +c.$$
Work Step by Step
Let $u=7^{-x}$, then $du=-7^{-x} \ln 7 dx$. We have
$$\int 7^{-x} dx=-\frac{1}{\ln 7}\int du=-\frac{1}{\ln 7}u+c\\
=-\frac{1}{\ln 7}u+c=-\frac{1}{\ln 7}7^{-x} +c.
$$
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