Linearity
If a linear system has a response to $ e^{2jt}\! $ of $ t*e^{-2jt}\! $ and a response to $ e^{-2jt}\! $ of $ t*e^{2jt}\! $, then it's response to $ cos(2t)\! $ must be $ t*cos(2t)\! $
To look at this in more detail, we must first understand that $ cos(2t)\! $ can be expressed as follows: $ \frac{1}{2}(e^{-2jt}+e^{2jt})\! $
Based on the given information, we know that the system must be linear. Since it is linear (and has the output shown in paragraph 1), we can conclude that it must have the output:
$ \frac{1}{2}(te^{-2jt}+te^{2jt})\! $.
Upon converting the output (above) back into a cosine function, we get the output:
$ t*cos(2t)\! $
