\[ l y(x)+y''(x)=0 \] ✓ Mathematica : cpu = 0.0038759 (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 = 0.02 (sec), leaf count = 21
\[y \relax (x ) = c_{1} \sin \left (\sqrt {l}\, x \right )+c_{2} \cos \left (\sqrt {l}\, x \right )\]