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!