\[ y'(x)=\frac {1}{2} \sqrt {a} \left (2 \sqrt {a x^4+8 y(x)}-\sqrt {a} x^3+2 x^3 \sqrt {a x^4+8 y(x)}+2 x^2 \sqrt {a x^4+8 y(x)}\right ) \] ✓ Mathematica : cpu = 0.547703 (sec), leaf count = 93
\[\left \{\left \{y(x)\to \frac {-1152 a c_1 x^4-1536 a c_1 x^3-4608 a c_1 x+2304 a c_1{}^2-288 a c_1+144 a x^8+384 a x^7+256 a x^6+1152 a x^5+1464 a x^4+96 a x^3+2304 a x^2+288 a x+9 a}{1152}\right \}\right \}\] ✓ Maple : cpu = 0.372 (sec), leaf count = 40
\[ \left \{ {\frac {1}{4}\sqrt {a{x}^{4}+8\,y \left ( x \right ) }}+{\frac {-3\,{x}^{4}-4\,{x}^{3}-12\,x}{12}\sqrt {a}}-{\it \_C1}=0 \right \} \]