Answer
$0$
Work Step by Step
Substitute $\frac{\pi}{6}$ to $x$ to obtain:
$\cos{(3x)}= \cos{[3(\frac{\pi}{6})]}=\cos{(\frac{\pi}{2})}$
$\dfrac{\pi}{2}$ is a special angle whose cosine value is $0$.
Thus, when $x=\frac{\pi}{6}$,
$\cos{(3x)} = 0$