\[ y'(x)=-\frac {1}{2} x^2 \left (a x-2 \sqrt {a \left (a x^4+8 y(x)\right )}\right ) \] ✓ Mathematica : cpu = 0.271347 (sec), leaf count = 34
\[\left \{\left \{y(x)\to \frac {1}{72} a \left (-96 c_1 x^3+144 c_1^2+16 x^6-9 x^4\right )\right \}\right \}\]
✓ Maple : cpu = 0.388 (sec), leaf count = 27
\[ \left \{ {\it \_C1}+{\frac {4\,a{x}^{3}}{3}}-\sqrt {a \left ( a{x}^{4}+8\,y \left ( x \right ) \right ) }=0 \right \} \]