\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) +a{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +by \left ( x \right ) =0} \]
Mathematica: cpu = 0.004501 (sec), leaf count = 58 \[ \left \{\left \{y(x)\to c_1 e^{\frac {1}{2} x \left (-\sqrt {a^2-4 b}-a\right )}+c_2 e^{\frac {1}{2} x \left (\sqrt {a^2-4 b}-a\right )}\right \}\right \} \]
Maple: cpu = 0.0 (sec), leaf count = 45 \[ \left \{ y \left ( x \right ) ={\it \_C1}\,{{\rm e}^{ \left ( -{\frac {a }{2}}+{\frac {1}{2}\sqrt {{a}^{2}-4\,b}} \right ) x}}+{\it \_C2}\,{ {\rm e}^{ \left ( -{\frac {a}{2}}-{\frac {1}{2}\sqrt {{a}^{2}-4\,b}} \right ) x}} \right \} \]