\[ \boxed { x \left ( \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2}-a \right ) {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) -y \left ( x \right ) \left ( \left ( y \left ( x \right ) \right ) ^{2}+{x}^{2}+a \right ) =0} \]
Mathematica: cpu = 0.040005 (sec), leaf count = 71 \[ \left \{\left \{y(x)\to \frac {1}{2} \left (c_1 x-\sqrt {-4 a+c_1^2 x^2+4 x^2}\right )\right \},\left \{y(x)\to \frac {1}{2} \left (\sqrt {-4 a+c_1^2 x^2+4 x^2}+c_1 x\right )\right \}\right \} \]
Maple: cpu = 0.078 (sec), leaf count = 112 \[ \left \{ \left ( \left ( y \left ( x \right ) \right ) ^{-2}- \left ( -{x }^{2}+a \right ) ^{-1} \right ) ^{-1}=-{x\sqrt {{x}^{2}-a}{\frac {1}{ \sqrt {{\it \_C1}+4\,{\frac {a}{{x}^{2}-a}}}}}}+{\frac {{x}^{2}}{2}}-{ \frac {a}{2}}, \left ( \left ( y \left ( x \right ) \right ) ^{-2}- \left ( -{x}^{2}+a \right ) ^{-1} \right ) ^{-1}={x\sqrt {{x}^{2}-a}{ \frac {1}{\sqrt {{\it \_C1}+4\,{\frac {a}{{x}^{2}-a}}}}}}+{\frac {{x}^ {2}}{2}}-{\frac {a}{2}} \right \} \]