Answer
$y=\frac{3\pi}{4}$
Work Step by Step
RECALL:
$y=\cos^{-1}{x} \longrightarrow \cos{y}=x$, $y$ is in the interval $[0, \pi]$
Note that $\cos {(\frac{3\pi}{4})}=-\frac{\sqrt2}{2}$.
Therefore,
$y=\cos^{-1}{(-\frac{\sqrt2}{2})}\longrightarrow y=\frac{3\pi}{4}$
