@Chrisir and @glv have provided some good advice and information for you to use.
It may be a good idea for you to divide your solution into parts. For example, you could write a function that takes two arrays of 20 integers each and returns the specifications for the circle, which would be the x and y coordinates of the center and the radius.
See Wikipedia: Euclidean distance. After you compute the coordinates of the center, that article may help you compute the distances of the clicked points to that center.
We look forward to seeing your code, formatted of course, as you develop it.