If you've studied polar coordinates, then this is easy. If not...
Have you studied trigonometry yet?
I have studied trig, I'm a college graduate with a BS in Computer Science, I've exhausted the extent of my current trig knowledge, I've gotta as far as knowing the length of the chord between P and P' but I have no way of knowing the angles of the right triangle created using the chord as a hypotenuse. Unless there is a way that I just don't remember?
I'll read into polar coordinates