\[
\boxed{
 \left\{ {\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}x \left( t \right) =ax \left( t \right) +by \left( t \right) ,{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}y \left( t \right) =cx \left( t \right) +dy \left( t \right)  \right\} }
\]