$ x(t)=e^{2jt} \to sys \to y(t)=te^{-2jt} $
$ x(t)=e^{-2jt} \to sys \to y(t)=te^{2jt} $
This implies that if $ x(t)=cos(2t) $ then $ y(t)=tcos(-2t)=tcos(2t) $.
