\[
\boxed{
 \left\{ a{\frac {\rm d}{{\rm d}t}}x \left( t \right) +b{\frac {\rm d}{{\rm d}t}}y \left( t \right) =\alpha\,x \left( t \right) +\beta\,y \left( t \right) ,b{\frac {\rm d}{{\rm d}t}}x \left( t \right) -a{\frac {\rm d}{{\rm d}t}}y \left( t \right) =\beta\,x \left( t \right) -\alpha\,y \left( t \right)  \right\} }
\]