x1^2a+x1b+c=y1
x2^2a+x2b+c=y2
x3^2a+x3b+c=y3
D=x1^2*x2-x1^2*x3-x2^2*x1+x2^2*x3+x3^2*x1-x3^2*x2
D1=y1*x2-y1*x3-y2*x1+y2*x3+y3*x1-y3*x2
D2=x1^2*y2-x1^2*y3-x2^2*y1+x2^2*y3+x3^2*y1-x3^2*y2
D3=x1^2*y3*x2-x1^2*y2*x3-x2^2*y3*x1+x2^2*y1*x3+x3^2*y2*x1-x3^2*y1*x2
A=D1/D
B=D2/D
C=D3/D