If that's the case, then
You find the critical points by setting
So you should solve
When you get your  critical points, set up a table [which should contain 4 intervals]. Pick a point within each of the intervals you set up and plug them into the derivative. The derivative will either have a positive or negative value at that point. Positive tells you that the function is increasing, and negative tells you that the function is decreasing.
Can you take it from here?