How do you evaluate Arctan -sqrt3?

$\frac{2 \pi}{3} \mathmr{and} \frac{5 \pi}{3}$
$\tan x = - \sqrt{3}$
arc $x = \frac{2 \pi}{3}$
Trig unit circle gives another arc $x = \frac{2 \pi}{3} + \pi = \frac{5 \pi}{3}$ that has the same tan value.