We are given $dv = \sqrt{x-20} \, dx$, so $v = \frac{2}{3}(x-20)^{3/2}$.
Using integration by parts, we have
$$\int u \, dv = uv - \int v \, du$$
$$\int x\sqrt{x-20} \, dx = x \cdot \frac{2}{3}(x-20)^{3/2} - \int \frac{2}{3}(x-20)^{3/2} \, dx$$
$$=
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