\[ l y(x)+y''(x)=0 \] ✓ Mathematica : cpu = 0.0037287 (sec), leaf count = 28
\[\left \{\left \{y(x)\to c_1 \cos \left (\sqrt {l} x\right )+c_2 \sin \left (\sqrt {l} x\right )\right \}\right \}\] ✓ Maple : cpu = 0.027 (sec), leaf count = 21
\[ \left \{ y \left ( x \right ) ={\it \_C1}\,\sin \left ( \sqrt {l}x \right ) +{\it \_C2}\,\cos \left ( \sqrt {l}x \right ) \right \} \]