Need help with this problem:

1) A plane flying 300 miles from Detroit to Chicago has been put in a holding pattern above the Chicago Airport. The holding Pattern is a circle with a diameter of 20 miles that the plane completes every 15 minutes. COmplete the following steps to write n equation that models the distance of the plane from Detroit as a function of time.

a) What is the period of this motion? Suggested ans by me : 2pi/15

b) What vertical stretch is needed to show the diameter of 20 miles? 310

c) what vertical shift is needed to show the 300 miles distance between the airports? 300

d) Write an equation modeling the plane's distance from Detroit while it is flying in this holding pattern.(sketch the equation)

Suggested ans: D(t) = -10cos (2pi/15)t + 300