Answer
$e^{mx}\left( m\cos {n}x-n\sin nx\right) $
Work Step by Step
$\dfrac {d}{dx}\left( e^{mx}\cos nx\right) =\left( \dfrac {d}{dx}\left( e^{mx}\right) \right) \times \cos nx+\left( \dfrac {d}{dx}\left( \cos nx\right) \right) \times e^{mx}=m\cos {nx}e^{mx}-n\sin nxe^{mx}=e^{mx}\left( m\cos {n}x-n\sin nx\right) $
