\[
\boxed{
 \left\{ t{\frac {\rm d}{{\rm d}t}}x \left( t \right) =2\,x \left( t \right) -t,{t}^{3}{\frac {\rm d}{{\rm d}t}}y \left( t \right) =-x \left( t \right) +{t}^{2}y \left( t \right) +t,{t}^{4}{\frac {\rm d}{{\rm d}t}}z \left( t \right) =-x \left( t \right) -{t}^{2}y \left( t \right) +{t}^{3}z \left( t \right) +t \right\} }
\]