You may want to review distances first.

As you may remember from high-school geometry, a circle is
determined by a center and a radius. Now, you can write the
equation for a circle given the center and radius. For example,
suppose you want the circle centered at
(−4 ⁄ 7, −6) with radius √3.
Then the equation is (*x* − (−4) / 7)^{2}
+ (*y* − (−6))^{2} −
(√3)^{2} = 0. Of course, we would probably rewrite
this as (*x* + 4 ⁄ 7)^{2} +
(*y* + 6))^{2} − 3 = 0.

As always, remember the minus signs; they're important!