Answer
$\frac{3\pi}{4}$
Work Step by Step
RECALL:
Since cosine and arccosine are inverse functions of each other, then, for all valid values of $x$,
$\arccos{(\cos(x))}=x$
Thus,
$\arccos{(\cos{(\frac{3\pi}{4})})}=\frac{3\pi}{4}$