\[ \boxed { x{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) + \left ( x+b \right ) {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) +ay \left ( x \right ) =0} \]
Mathematica: cpu = 0.030504 (sec), leaf count = 40 \[ \left \{\left \{y(x)\to c_1 e^{-x} U(b-a,b,x)+c_2 e^{-x} L_{a-b}^{b-1}(x)\right \}\right \} \]
Maple: cpu = 0.062 (sec), leaf count = 33 \[ \left \{ y \left ( x \right ) ={\it \_C1}\,{{\rm e}^{-x}}{{\sl M}\left (- a+b,\,b,\,x\right )}+{\it \_C2}\,{{\rm e}^{-x}}{{\sl U}\left (-a+b,\,b ,\,x\right )} \right \} \]