$($CTFT and Properties for LTI Systems$)(20$ points$)$ Consider an LTI system whose response to the input x$($t$)=($e$^-$t$+$e$^-3$t$)$u$($t$)$ is y$($t$)=(2$e$^-$t $-2$e$^-4$t$)$u$($t$)$