Answer
$\frac{2\pi}{3}$
Work Step by Step
$\cot^{-1}{-\frac{\sqrt3}{3}}=\frac{2\pi}{3}$, because $\cot{\frac{2\pi}{3}}=-\frac{\sqrt3}{3}$ and $\frac{2\pi}{3}$ is in the range of $\cot^{-1}{x}$, which is $(0,\pi).$
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