Answer
$\dfrac{\sqrt {3}}{2}$
Work Step by Step
Recall the rule: $(f \circ \ h ) (x) =f[h(x)]$
Let us consider that $f(x)=\sin x $ and $h(x)= 2 x$
We know from the unit circle that $\sin (\dfrac{\pi}{3}) =\dfrac{\sqrt {3}}{2}$
Thus, we have: $(f \ \circ \ h ) (\dfrac{\pi}{6}) =f[h(\dfrac{\pi}{6})] \\=
f [2 \cdot (\dfrac{\pi}{6})] \\=f(\dfrac{\pi}{3}) \\=\sin (\dfrac{\pi}{3}) \\= \dfrac{\sqrt {3}}{2}$
