\[
\boxed{
 \left\{ {\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}x \left( t \right) ={\it a1}\,x \left( t \right) +{\it b1}\,y \left( t \right) +{\it c1},{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}y \left( t \right) ={\it a2}\,x \left( t \right) +{\it b2}\,y \left( t \right) +{\it c2} \right\} }
\]