Find a line intersecting a known line at right angle, given a point

Question

This is basic graphics geometry and/or trig, and I feel dumb for asking it, but I can\'t remember how this goes. So:

1. I have a line defined by two points (x1, y1

Answer 1

The answer line is:

y=ax+b
where a=(x1-x2)/(y2-y1)
      b=yp-(x1-x2)*xp/(y2-y1)

How the result was obtained:

1) slope for the original line:   (y2-y1)/(x2-x1)

2) slope for the answer: -1/((y2-y1)/(x2-x1)) = (x1-x2)/(y2-y1)

3) Plug this into (xp,yp) we can have the result line.

Just calculate the answer from the lines after this (this is too long... I am hungry).