\[ \boxed { \left \{ {\frac {\rm d}{{\rm d}t}}x \left ( t \right ) =ax \left ( t \right ) ,{\frac {\rm d}{{\rm d}t}}y \left ( t \right ) =b \right \} } \]
Mathematica: cpu = 0.005001 (sec), leaf count = 22 \[ \left \{\left \{x(t)\to c_1 e^{a t},y(t)\to b t+c_2\right \}\right \} \]
Maple: cpu = 0.015 (sec), leaf count = 19 \[ \left \{ \left \{ x \left ( t \right ) ={\it \_C1}\,{{\rm e}^{at}},y \left ( t \right ) =bt+{\it \_C2} \right \} \right \} \]