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 *x*^{2}
+ *y*^{2} + 2*x* + 4*y* + 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 *x*^{2} + 2*x* + 1 = (*x* +
1)^{2}. The terms involving *y* are as well,
namely *y*^{2} + 4*y* + 4 = (*y* +
2)^{2}. The sum of these would come out
to *x*^{2} + *y*^{2} + 2*x* +
4*y* + 5. Now, the left-hand side is
actually *x*^{2} + *y*^{2} + 2*x* +
4*y* + 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 *x*^{2} + *ax*, then you can complete it
to *x*^{2} + *ax* + *a*^{2} / 4 =
(*x* + *a* / 2)^{2}.