You may want to review the standard form for equations of circles first.
Sometimes, the equation will be given to you in a form you're more used to for polynomials, like x2 + y2 + 2x + 4y + 4 = 0. You can put it into the usual form for circles by a process called completing the square. The idea is that the terms involving x are the beginning of a square polynomial, namely x2 + 2x + 1 = (x + 1)2. The terms involving y are as well, namely y2 + 4y + 4 = (y + 2)2. The sum of these would come out to x2 + y2 + 2x + 4y + 5. Now, the left-hand side is actually x2 + y2 + 2x + 4y + 4, so that's (x + 1)2 + (y + 2)2 − 1 (since 4 = 5 − 1). So we have the equation (x + 1)2 + (y + 2)2 − 1 = 0, which represents a circle with center (−1, −2) and radius 1.
This works in general. If a polynomial starts with x2 + ax, then you can complete it to x2 + ax + a2 / 4 = (x + a / 2)2.