\[ \boxed { \left ( {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) \right ) y \left ( x \right ) -a \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{2}=0} \]
Mathematica: cpu = 0.034504 (sec), leaf count = 26 \[ \left \{\left \{y(x)\to c_2 \left (-a x-c_1+x\right ){}^{\frac {1}{1-a}}\right \}\right \} \]
Maple: cpu = 1.638 (sec), leaf count = 25 \[ \left \{ y \left ( x \right ) = \left ( {\frac {1}{ \left ( 1-a \right ) \left ( {\it \_C1}\,x+{\it \_C2} \right ) }} \right ) ^{ \left ( a-1 \right ) ^{-1}} \right \} \]