\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) +ly \left ( x \right ) =0} \]
Mathematica: cpu = 0.004001 (sec), leaf count = 28 \[ \left \{\left \{y(x)\to c_2 \sin \left (\sqrt {l} x\right )+c_1 \cos \left (\sqrt {l} x\right )\right \}\right \} \]
Maple: cpu = 0.016 (sec), leaf count = 21 \[ \left \{ y \left ( x \right ) ={\it \_C1}\,\sin \left ( \sqrt {l}x \right ) +{\it \_C2}\,\cos \left ( \sqrt {l}x \right ) \right \} \]