\[ \boxed { 2\, \left ( {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) \right ) y \left ( x \right ) - \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{2}+a=0} \]
Mathematica: cpu = 0.005501 (sec), leaf count = 31 \[ \left \{\left \{y(x)\to \frac {x^2 \left (c_1^2-a\right )}{4 c_2}+c_1 x+c_2\right \}\right \} \]
Maple: cpu = 1.622 (sec), leaf count = 24 \[ \left \{ y \left ( x \right ) ={\frac { \left ( {{\it \_C1}}^{2}-a \right ) {x}^{2}}{4\,{\it \_C2}}}+{\it \_C1}\,x+{\it \_C2} \right \} \]