\[ y(x)^3 y''(x)-a=0 \] ✓ Mathematica : cpu = 1.44388 (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 = 0.495 (sec), leaf count = 46
\[ \left \{ y \left ( x \right ) ={\frac {1}{{\it \_C1}}\sqrt {{\it \_C1}\, \left ( \left ( {\it \_C2}+x \right ) ^{2}{{\it \_C1}}^{2}+a \right ) }},y \left ( x \right ) =-{\frac {1}{{\it \_C1}}\sqrt {{\it \_C1}\, \left ( \left ( {\it \_C2}+x \right ) ^{2}{{\it \_C1}}^{2}+a \right ) }} \right \} \]