Let's take a look at the graph of the polynomial:
Now, notice that those three turning points tell us that the function changes from decreasing to increasing, or viceversa, right at that point.
This way, the decreasing intervals are:
[tex]\begin{gathered} (-\infty,-2.531) \\ (-0.136,1.398) \\ \end{gathered}[/tex]
And the increasing intervals are:
[tex]\begin{gathered} (-2.531,-0.136) \\ (1.398,\infty) \end{gathered}[/tex]
