Answer
$f(t)=(t+3) e^{-t}$
Work Step by Step
We are given that $x=t-3$
Re-write as: $t=x+3$
When $a$ has a positive value then , shift the function to the right for $a$ units and when $a$ has a negative value then , shift the function to the left for $a$ units
Now, $f(t-3) =t e^{-(t-3)} $ and $f(x)=(x+3) e^{-x}$
So, we have the independent variable $t$ is: $f(t)=(t+3) e^{-t}$
