y=kx+m,
y1=kx1+m
y2=kx2+m,
k=(y2-y1)/(x2-x1)
m=y1-(y2-y1)*x1/(x2-x1)
y=(y2-y1)/(x2-x1)x+(y1*(x2-x1)-x1(y2-y1))/(x2-x1)
(y1-y2)x+(x2-x1)y+(x1y2-x2y1)=0,
a=y1-y2,
b=x2-x1,
c=x1y2-x2y1,