Answer
$-21 \cos (\dfrac{\theta}{3})+C$
Work Step by Step
Given: $\int 7 \sin (\dfrac{\theta}{3}) d\theta$
Thus, $\int 7 \sin (\dfrac{\theta}{3}) d\theta=7(-3) \cos (\dfrac{\theta}{3}) +C=-21 \cos (\dfrac{\theta}{3})+C$
and
$\dfrac{d}{d \theta}(-21 \cos (\dfrac{\theta}{3})+C)=21 \sin (\dfrac{\theta}{3})(\dfrac{1}{3}=7 \sin (\dfrac{\theta}{3})$
