The response to cos(2t) can be broken up into to parts because $ cos(2t) = \frac{e^{2jt} + e^{-2jt}}{2} $.
The response due to $ \ e^{2jt} $ is $ \ te^{-2jt} $
The response due to $ \ e^{-2jt} $ is $ \ te^{2jt} $
Placing the responses into the equation we get $ \frac{te^{-2jt} + te^{2jt}}{2} $ which is equal to $ \ tcos(2t) $.
