Answer
$\sqrt 3$
Work Step by Step
Let us consider:
$f(\theta)=\sin \theta$ and $\theta=60^{\circ}$
We know from the unit circle that
$\sin (60^{\circ})=\sin (\dfrac{\pi}{3})=\dfrac{\sqrt 3}{2}$
Thus, we have: $ 2 \ f (\theta)= 2 \times \sin (60^{\circ})
\\=2 \times \sin (\dfrac{\pi}{3}) \\ = 2 \times \dfrac{\sqrt 3}{2} \\=\sqrt 3$