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.