\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) +a{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +by \left ( x \right ) -f \left ( x \right ) =0} \]
Mathematica: cpu = 0 (sec), leaf count = 0 \[ \text {Hanged} \]
Maple: cpu = 0.078 (sec), leaf count = 128 \[ \left \{ y \left ( x \right ) ={{\rm e}^{ \left ( -{\frac {a}{2}}+{\frac {1}{2}\sqrt {{a}^{2}-4\,b}} \right ) x}}{\it \_C2}+{{\rm e}^{ \left ( -{ \frac {a}{2}}-{\frac {1}{2}\sqrt {{a}^{2}-4\,b}} \right ) x}}{\it \_C1} +{1 \left ( \int \!f \left ( x \right ) {{\rm e}^{-{\frac {x}{2} \left ( - a+\sqrt {{a}^{2}-4\,b} \right ) }}}\,{\rm d}x{{\rm e}^{x\sqrt {{a}^{2}- 4\,b}}}-\int \!f \left ( x \right ) {{\rm e}^{{\frac {x}{2} \left ( a+ \sqrt {{a}^{2}-4\,b} \right ) }}}\,{\rm d}x \right ) {{\rm e}^{-{\frac { x}{2} \left ( a+\sqrt {{a}^{2}-4\,b} \right ) }}}{\frac {1}{\sqrt {{a}^{ 2}-4\,b}}}} \right \} \]