A system is said to be linear if it follows the following conditions
1) The response to $ x_1(t) $ + $ x_2(t) $ is $ y_1(t) $ +$ y_2(t) $.
2) The response to $ ax_1(t) $ is $ ay_1(t) $, where a is any complex constant.
Example for a linear system is
$ x_1 $ = 8$ e^t $
$ x_2 $=8$ t^2 $
Let ,
$ x_3 $ = 5$ e^t $ + 3$ t^2 $
The output is
$ y(t) $ = 8$ x_3 $
$ y(t) $=40 $ e^t $ + 24 $ t^2 $
$ y(t) $= 5$ x_1 $ + 3 $ x_2 $
