Use the distance formula. √(x-x)^2 + (y-y)^2 Subtract the two x coordinates from each other and square them, Subtract the two y coordinates and square them. Add them together and then find the square root of that. This will give you the distance between the two points which for you is also the radius.
One thing to remember is to keep the order the same when you are using the coordinates. Most people use the second set of coordinates first. so the x from on the circle minus the x in the center. and then the y on the circle minus the y in the center. ( I hope that makes sense)
