\[ \boxed { 4\,{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) +9\,xy \left ( x \right ) =0} \]
Mathematica: cpu = 0.004501 (sec), leaf count = 42 \[ \left \{\left \{y(x)\to c_1 \text {Ai}\left (\sqrt [3]{-1} \left (\frac {3}{2}\right )^{2/3} x\right )+c_2 \text {Bi}\left (\sqrt [3]{-1} \left (\frac {3}{2}\right )^{2/3} x\right )\right \}\right \} \]
Maple: cpu = 0.016 (sec), leaf count = 29 \[ \left \{ y \left ( x \right ) ={\it \_C1}\,{{\rm Ai}\left (-{\frac {{3}^{ {\frac {2}{3}}}\sqrt [3]{2}x}{2}}\right )}+{\it \_C2}\,{{\rm Bi}\left (- {\frac {{3}^{{\frac {2}{3}}}\sqrt [3]{2}x}{2}}\right )} \right \} \]