Auxiliary equation: $$m^2 + am + b = 0$$
Roots $$m_1, m_2$$:
- Real & distinct: $$y = C_1 e^{m_1x} + C_2 e^{m_2x}$$
- Real & equal: $$y = (C_1 + C_2 x)e^{m_1x}$$
- Complex: $$m = \alpha \pm i\beta$$, solution: $$y = e^{\alpha x}(C_1 \cos \beta x + C_2 \sin \beta x)$$