title: How can I calculate coordinates of the perpendicular line?
body: <p>Lines (x1, y1), (x2, y2) and (x3, y3), (x4, y4) are perpendicular. I have coordinates of points (x1, y1), (x2, y2), (x3, y3) and length in pixels of a line (x3, y3), (x4, y4). I need to find coordinates of point (x4, y4). What is the pseudocode for calculating (x4, y4)?</p>
  <p><img src="https://i.stack.imgur.com/oB31k.png" alt="picture" /></p>