Answer
$\theta =-\dfrac {\pi }{2}+2\pi k$
Work Step by Step
$\sin ^{2}\theta -\sin \theta -2=0 \Rightarrow \sin \theta =\dfrac {1\pm \sqrt {1+4\times 2}}{2}=\dfrac {1\pm 3}{2}=2;-1 $
$ sin\theta $ can not be greater than 1 so
$\sin \theta =-1\Rightarrow \theta =-\dfrac {\pi }{2}+2\pi k$
