\[ y(x)^3 y''(x)-a=0 \] ✓ Mathematica : cpu = 1.85107 (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 = 0.131 (sec), leaf count = 70
\[ \left \{ y \left ( x \right ) ={\frac {1}{{\it \_C1}}\sqrt {{\it \_C1}\, \left ( {{\it \_C1}}^{2}{{\it \_C2}}^{2}+2\,{{\it \_C1}}^{2}{\it \_C2}\,x+{{\it \_C1}}^{2}{x}^{2}+a \right ) }},y \left ( x \right ) =-{\frac {1}{{\it \_C1}}\sqrt {{\it \_C1}\, \left ( {{\it \_C1}}^{2}{{\it \_C2}}^{2}+2\,{{\it \_C1}}^{2}{\it \_C2}\,x+{{\it \_C1}}^{2}{x}^{2}+a \right ) }} \right \} \]