\[
\boxed{
 \left\{ {\frac {\rm d}{{\rm d}t}}x \left( t \right) +{\frac {\rm d}{{\rm d}t}}y \left( t \right) +y \left( t \right) =f \left( t \right) ,{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}x \left( t \right) +{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}y \left( t \right) +{\frac {\rm d}{{\rm d}t}}y \left( t \right) +x \left( t \right) +y \left( t \right) =g \left( t \right)  \right\} }
\]