Answer
$y=\displaystyle \frac{2\pi}{3}$
Work Step by Step
By definition of arccos,
y is the number from $[0, \pi]$ such that $\displaystyle \cos y=-\frac{1}{2}.$
Using the unit circle, we find that
$\displaystyle \frac{2\pi}{3}\in[0, \pi] $and $\displaystyle \cos\frac{2\pi}{3}=-\frac{1}{2}.$
$y=\displaystyle \frac{2\pi}{3}$
