Identify the coordinates of the circle's center. Use the round button located just under the screen to scroll around the screen until you reach the point at the center of the circle. Check the bottom of the screen for the coordinates of the point, once your cursor has landed on top of it. An example coordinate might be (2,3).
Calculate the circle's radius. Move your cursor to the right of the center until you reach the furthest arc of the circle. For your circle centered at (2,3), for example, this might be to the point (7,3). If you move left to right, you will measure your radius along the x-axis; in this example, this would be 7-2, or r = 5.
Plug your (h,k) and "r" values into the radius form equation of a circle, (x - h)^2 + (y - k)^2 = r^2. For the example circle with a center at (2,3) and a radius of r = 5, this would be: (x - 2)^2 + (y - 3)^2 = 25.