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 $
