Since the graph of the equation is a circle a cannot be 0. So we will divide the equation by a to get the equation as

x^2 + y^2 + (b/a) x + (c/a) y + d/a = 0 For easy handling we may put b/a = P, c/a = Q and d/a = R. Thus the equation becomes

x^2 + y^2 + P x + Q y + R = 0 , Now we have three variables and three points and hence we can solve it.