You may want to review writing equations of circles first.

If you're given the equation of a circle, you can read off its center and radius. For example, consider the equation (x + 1 ⁄ 6)2 + (y − 1)2 − 16 = 0. Then the center is (−1 ⁄ 6, 1) and the radius is √16 = 4. Again, remember that the signs are switched from what they are in the equation.