Determine the point(s) (if any) at which the graph of the function has a horizontal tangent line.

http://i146.photobucket.com/albums/r...kitty216/3.jpg

First I plugged in o to all the x's and got the point (2,0).. I don't know how to find the other two points. I want to learn the method of how to work these types of problems out, then I can do the same types of problemsusing the same method. Help would be great!