title: Calculating the coordinates of the third point of a triangle
body: <p>OK, I know this sounds like it should be asked on math.stackoverflow.com, but this is embarrassingly simple maths that I've forgotten from high-school, rather than advanced post-graduate stuff!</p>
  
  <p>I'm doing some graphics programming, and I have a triangle. Incidentally, two of this triangle's sides are equal, but I'm not sure if that's relevant. I have the coordinates of two of the corners (vertices), but not the third (these coordinates are pixels on the screen, in case that's relevant). I know the lengths of all three sides.</p>
  
  <p>How do I determine the coordinates of the unknown vertex?</p>