\[
\boxed{
 \left\{ {\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}x \left( t \right) +ay \left( t \right) =0,{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}y \left( t \right) -{a}^{2}y \left( t \right) =0 \right\} }
\]