#### Answer

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

#### Work Step by Step

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