\[ \boxed { 8\,{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) +9\, \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{4}=0} \]
Mathematica: cpu = 0.034004 (sec), leaf count = 92 \[ \left \{\left \{y(x)\to c_2-\frac {1}{3} \sqrt [3]{-\frac {1}{3}} \left (9 x-8 c_1\right ){}^{2/3}\right \},\left \{y(x)\to \frac {\left (9 x-8 c_1\right ){}^{2/3}}{3 \sqrt [3]{3}}+c_2\right \},\left \{y(x)\to \frac {(-1)^{2/3} \left (9 x-8 c_1\right ){}^{2/3}}{3 \sqrt [3]{3}}+c_2\right \}\right \} \]
Maple: cpu = 0.062 (sec), leaf count = 51 \[ \left \{ y \left ( x \right ) = \left ( {\it \_C1}+x \right ) ^{{\frac {2} {3}}}+{\it \_C2},y \left ( x \right ) ={\frac {i\sqrt {3}-1}{2} \left ( { \it \_C1}+x \right ) ^{{\frac {2}{3}}}}+{\it \_C2},y \left ( x \right ) = -{\frac {i\sqrt {3}+1}{2} \left ( {\it \_C1}+x \right ) ^{{\frac {2}{3}} }}+{\it \_C2} \right \} \]