# The differential equation obtained by eliminating $a$ and $b$ from $y=ae^{3x}+be^{-3x}$ is
$\begin{array}{1 1}(1)\frac{d^{2}y}{dx^{2}}+ay=0&(2)\frac{d^{2}y}{dx^{2}}-9y=0\\(3)\frac{d^{2}y}{dx^{2}}-9\frac{dy}{dx}=0&(4)\frac{d^{2}y}{dx^{2}}+9x=0\end{array}$