I think you forgot something in your calculation of the derivative.
The derivative y'(t) is the rate of change of y(t). In other words, how fast/slow y(t) is increasing. You just need to substitute 15 for t in your equation for y'(t) to get the answer to the first question.
For the second question, you want the maximum possible value of y'(t). You can probably just figure out the answer directly, but you also probably know how to find the maximum value for a function. In this case you find where the derivative is zero, so the derivative of y'(t) is y''(t).
If you think about the problem physically, you should be able to verify your answer. Where does a ferris wheel go fastest in the up-down direction?
Let us know if you run into any trouble.