Chapter 4.3, Problem 27E

In Exercises 27-32, find the values of $x$ at which the function has a possible relative maximum or minimum point. (Recall that $e^x$ is positive for all $x$) Use the second derivative to determine the nature of the function at these points.

$f\left(x\right)=\left(1+x\right)e^{-3x}$