Answer
$\frac{1}{48}(x^3+9)^{16}+c$
Work Step by Step
Let $u=x^3+9$, then $du=3x^2dx$. Hence, we have
$$\int x^2(x^3+9)^{15}dx=\frac{1}{3}\int u^{15}du\\
=\frac{1}{3}\frac{1}{16}u^{16}+c=\frac{1}{48}(x^3+9)^{16}+c.$$
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