As always, start with a Free-Body Diagram. I will choose a +y direction upward. There is a weight on the coaster, w, straight down. There is an upward (presumably a normal) force equal to 2w acting straight up. That's it for the vertical direction.
So what is the net force on any object moving in a circular path? Right! It's the centripetal force. Thus
I will leave you with the rest.