I think on RHK 4.54 we have to consider the gravity. Why did you not add 9.8m/s^2 to 4.11m/s^2 for (a) and 4.11m/s^2 - 9.8m/s^2?
The person on the carnival ride is performing uniform circular motion, if you sketch how their velocity vector changes in time you will find that their acceleration is simply the centripetal acceleration. When we start working on free body diagrams, you should see that the force due to gravity is exactly canceled by some force on the person exerted by their chair on the ride; however, again, this problem can be solved just from the vector-based definition of acceleration and the information given in the problem.