\[ y'(x)=\frac {1}{512} x \left (a^3 x^{12}+24 a^2 x^8 y(x)+8 a^2 x^8+192 a x^4 y(x)^2+128 a x^4 y(x)-256 a x^2+512 y(x)^3+512 y(x)^2+512\right ) \] ✓ Mathematica : cpu = 0.199203 (sec), leaf count = 1
\[\{\}\]
✓ Maple : cpu = 0.064 (sec), leaf count = 40
\[ \left \{ y \left ( x \right ) =-{\frac {a{x}^{4}}{8}}-{\frac {1}{3}}+{\frac {29\,{\it RootOf} \left ( {x}^{2}-162\,\int ^{{\it \_Z}}\! \left ( 841\,{{\it \_a}}^{3}-27\,{\it \_a}+27 \right ) ^{-1}{d{\it \_a}}+6\,{\it \_C1} \right ) }{9}} \right \} \]