Answer
$y=\frac{2\pi}{3}$
Work Step by Step
RECALL:
$y=\arccos{x} \longrightarrow \cos{y}=x$
Note that:
$\cos{(\frac{2\pi}{3})} = -\frac{1}{2}$
Thus,
$y=\arccos{(-\frac{1}{2})}\longrightarrow y=\frac{2\pi}{3}$
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