Answer
$\frac{5\pi}{4}$
Work Step by Step
$cot(\frac{2\theta}{3})=-\sqrt 3$,
$\frac{2\theta}{3}=k\pi+\frac{5\pi}{6}$
$\theta=\frac{3k\pi}{2}+\frac{5\pi}{4}$ (k is an integer),
$\theta=\frac{5\pi}{4}$ in $[0,2\pi)$.
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