\[ \boxed { x{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) + \left ( b-x \right ) {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) -ay \left ( x \right ) =0} \]
Mathematica: cpu = 0.021503 (sec), leaf count = 24 \[ \left \{\left \{y(x)\to c_1 U(a,b,x)+c_2 L_{-a}^{b-1}(x)\right \}\right \} \]
Maple: cpu = 0.063 (sec), leaf count = 17 \[ \left \{ y \left ( x \right ) ={\it \_C1}\,{{\sl M}\left (a,\,b,\,x \right )}+{\it \_C2}\,{{\sl U}\left (a,\,b,\,x\right )} \right \} \]