\[
\boxed{
 \left\{ {\frac {\rm d}{{\rm d}t}}x \left( t \right) = \left( ay \left( t \right) +b \right) x \left( t \right) ,{\frac {\rm d}{{\rm d}t}}y \left( t \right) = \left( cx \left( t \right) +d \right) y \left( t \right)  \right\} }
\]