The radius is the distance from the center to any point. The distance from (2,1) to (0,1) is of course 2. The general formula for a circle with radius r and center (a,b) is

In our case we get
First you would need to find out how many 9 payments of 58$ would be and that is 522. Then you would need to subtract the 522 and the 480 and your answer would be 42$ so the interest would be 42$