`int e^(-x) dx, u = -x` Evaluate the integral by making the given substitution.

You need to use the following substitution `-x = u` , such that:

`-x = u=> -dx = du => dx = -du`

`int e^(-x) dx = -int e^u du = -e^u + c`

Replacing back  `-x` for u yields:

`int e^(-x) dx = -e^(-x) + c`

Hence, evaluating the indefinite integral, yields` int e^(-x) dx = -e^(-x) + c`