If $y=e^{6x}\cos 3x$. What is $\frac{d^2y}{dx^2}$?

$9e^{3x}(3\cos 4x-4\sin 3x)$ $9e^{6x}(3\cos 3x-4\sin 3x)$ $3e^{6x}(\cos 3x-2\sin 3x)$ $e^{6x}(3\cos 3x-\sin 3x)$

$9e^{6x}(3\cos 3x-4\sin 3x)$

$\frac{dy}{dx}=e^{6x}(-3\sin 3x+6\cos 6x)$. Differentiating again w.r.to x we get $\frac{d^2y}{dx^2}=9e^{6x}(3\cos 3x-4\sin 3x)$