Answer
$$\sin (\theta+\pi) =-\sin \theta$$
Work Step by Step
Since
\begin{aligned}
\sin (\theta+\pi) &=\sin \theta \cos \pi+\cos \theta \sin \pi \\
&=\sin \theta(-1)+\cos \theta(0) \\
\end{aligned}
Then $$\sin (\theta+\pi) =-\sin \theta$$