\[ y(x)^3 y''(x)-a=0 \] ✓ Mathematica : cpu = 1.48374 (sec), leaf count = 88
\[\left \{\left \{y(x)\to -\frac {\sqrt {a+c_1{}^2 x^2+2 c_2 c_1{}^2 x+c_2{}^2 c_1{}^2}}{\sqrt {c_1}}\right \},\left \{y(x)\to \frac {\sqrt {a+c_1{}^2 x^2+2 c_2 c_1{}^2 x+c_2{}^2 c_1{}^2}}{\sqrt {c_1}}\right \}\right \}\] ✓ Maple : cpu = 0.298 (sec), leaf count = 46
\[y \relax (x ) = \frac {\sqrt {\left (\left (x +c_{2}\right )^{2} c_{1}^{2}+a \right ) c_{1}}}{c_{1}}\]