\[ \boxed { {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) ={\frac {2\,a+{x}^{2}\sqrt {- \left ( y \left ( x \right ) \right ) ^{2}+4\,ax}}{y \left ( x \right ) }}=0} \]
Mathematica: cpu = 2.294291 (sec), leaf count = 101 \[ \left \{\left \{y(x)\to -\frac {\sqrt {147456 a^7 x-4096 a^6 x^6+128 a^3 e^{c_1} x^3-e^{2 c_1}}}{192 a^3}\right \},\left \{y(x)\to \frac {\sqrt {147456 a^7 x-4096 a^6 x^6+128 a^3 e^{c_1} x^3-e^{2 c_1}}}{192 a^3}\right \}\right \} \]
Maple: cpu = 0.125 (sec), leaf count = 27 \[ \left \{ -\sqrt {- \left ( y \left ( x \right ) \right ) ^{2}+4\,ax}-{ \frac {{x}^{3}}{3}}-{\it \_C1}=0 \right \} \]