If $y=5cosx-3sinx,$ then $\frac{d^2y}{dx^2}$ is equal to :

$y$ $-y$ $2y$ $-2y$

$-y$

The correct answer is Option (2) → $-y$ $y=5\cos x-3\sin x$ $y'=-5\sin x-3\cos x$ $y''=-5\cos x+3\sin x=-y$ so $y''=-y$