\[ y'(x)=-\frac {1}{2} x^2 \left (a x-2 \sqrt {a \left (a x^4+8 y(x)\right )}\right ) \] ✓ Mathematica : cpu = 0.410196 (sec), leaf count = 34
\[\left \{\left \{y(x)\to \frac {1}{72} a \left (16 x^6-9 x^4-96 c_1 x^3+144 c_1{}^2\right )\right \}\right \}\] ✓ Maple : cpu = 0.476 (sec), leaf count = 27
\[\left \{\frac {4 a \,x^{3}}{3}+c_{1}-\sqrt {\left (a \,x^{4}+8 y \left (x \right )\right ) a} = 0\right \}\]