With a function graph? ! ? describe how to find the interval(s) where the function is decreasing and how interval notation is used to express the interval(s). i really don't understand.

We notice that as you go from -5 to 0, the line is continuously decreasing. Here we say that in the interval (-5, 0) the function is "decreasing".

Similarly, as we go (in x) from 0 to 4, the line goes up. Here we say that the function is increasing in (0, 4).

In [-8, -5) and [4, 8) the line is "constant", so no increase or decrease at any point.

Using a square bracket " [ " indicates that the endpoint is included, while a " ( " indicates it is not. 

When we indicate increasing or decreasing intervals the endpoints are always open, that is, not included. This is a subtle point related to a later concept.