Equation of a line y=mx+b passing through two points (x1,y1) and (x2,y2) fails on x1=x2
Perpendicular y=mx+b to a line through a point (x,y) not on the line
Intersection point of two lines y=m1x+b1 and y=m2x+b2
Mirror point (x1,y1) about perpendicular point (x2,y2) on a line
Wrapping full function mirrors point x,y about line through x1,y1 and x2,y2