title: How to get the vector between two vectors?
body: <p>I have three points with x and y values to start with. What I actually want is the position where the actual vector would go (look at image provided). 
  Can you help me ? I tried around a little bit with atan2 and parallelograms, but unfortunately without success. </p>
  
  <p><img src="https://i.stack.imgur.com/wN1S7.png" alt="Image"></p>