#### Answer

$\dfrac {mx\times \cos \left( mx\right) -\sin mx}{x^{2}}$

#### Work Step by Step

$\dfrac {d}{dx}\left( \dfrac {\sin mx}{x}\right) =\dfrac {\left( \dfrac {d}{dx}\left( \sin mx\right) \right) \times x-\left( \dfrac {d}{dx}\left( x\right) \right) \times \sin mx}{x^{2}}=\dfrac {mx\times \cos \left( mx\right) -\sin mx}{x^{2}}$