Math cosine problem PLEASE HELP?

The radius of 50 feet means that the amplitude is 50. The lowest point is 5 feet above the ground, but the center is 50 feet above those 5 feet, so you have to add 55 to the entire function. The period is 36 seconds, so 36=(2Pi)/b, and you can solve that to get b=Pi/18. So, I think the answer is y=50cos(Pi/18 times x)+55. However, I am not 100% sure if this is correct because the rider is starting at the minimum height.