#### Answer

$=\dfrac {\sqrt {2}}{2}\left( \cos x+\sin x\right) $

#### Work Step by Step

$\sin \left( \dfrac {3\pi }{4}-x\right) =\sin \dfrac {3\pi }{4}\cos x-\cos \dfrac {3\pi }{4}\sin x=\dfrac {\sqrt {2}}{2}\cos x-\left( -\dfrac {\sqrt {2}}{2}\sin x\right) =\dfrac {\sqrt {2}}{2}\left( \cos x+\sin x\right) $