Find the general solutions of the following equations:
$\mathbf{a} \frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}+5 \frac{\mathrm{d} y}{\mathrm{~d} x}+6 y=0$
$\mathbf{b} \frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}+4 \frac{\mathrm{d} y}{\mathrm{~d} x}+4 y=0$
$\mathbf{c} \frac{\mathrm{d}^{2} y}{\mathrm{~d} x^{2}}-2 \frac{\mathrm{d} y}{\mathrm{~d} x}+4 y=0$