The general solution of $\dfrac{\mathrm{d^{2}} y}{\mathrm{d} x^{2}}-6\dfrac{\mathrm{d} y}{\mathrm{d} x}+9y=0$ is
1. $y=C_{1}e^{3x}+C_{2}e^{-3x}$
2. $y=(C_{1}+C_{2}x)e^{-3x}$
3. $y=(C_{1}+C_{2}x)e^{3x}$
4. $y=C_{1}e^{3x}$