If X1(t) -> system => y1(t)
and X2(t) -> system => y2(t)
implies a*X1(t)+ b*X2(t) -> system => a*y1(t) + b*y2(t)
for any complex number a,b
then the system is called linear.
E.g. y(t)=$ x^2 (t) $ is non-linear y(t)=2(x)t is linear
