$\begin{align}  & y=3x\sin x\cos x \\ & \frac{dy}{dx}=3x\sin x\frac{d}{dx}(\cos x)+3x\cos x\frac{d}{dx}(\sin x)+\sin x\cos x\frac{d}{dx}(3x) \\ & \frac{dy}{dx}=3x\sin x(-\sin x)+3x\cos x(\cos x)+\sin x\cos x(3) \\ & \frac{dy}{dx}=-3x{{\sin }^{2}}x+3x{{\cos }^{2}}x+3\sin x\cos x \\\end{align}$