Solve:
$\mathbf{a} \frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}+3 \frac{\mathrm{d} y}{\mathrm{~d} x}+2 y=0$
$\mathbf{b} \frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}-7 \frac{\mathrm{d} y}{\mathrm{~d} x}+6 y=0$
$\mathbf{c} \frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}-6 \frac{\mathrm{d} y}{\mathrm{~d} x}+9 y=0$
d $\frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}-4 \frac{\mathrm{d} y}{\mathrm{~d} x}+5 y=0$
e $\frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}-8 \frac{\mathrm{d} y}{\mathrm{~d} x}+16 y=0$