Definition
If
$ x_{1}(t) \to sys \to *a \to $ $ + \to a x_{1}(t) + b x_{2}(t) $ $ x_{2}(t) \to sys \to *b \to $
And
$ x_{1}(t) \to *a \to $ $ + \to sys \to a x_{1}(t) + b x_{2}(t) $ $ x_{2}(t) \to *b \to $
And $ a $ and $ b $ are any complex number,
Then the system is linear.
