http://www.wikihow.com/Find-the-Angle-Between-Two-Vectors

Or maybe use the

angleBetween() 

thank you janjacobsv .first link is not my way :) 

Have you tried? maybe you can show your code and I can check what goes wrong.

It can be a bit tricky with vectors, You have three vectors discribing point in space, e.g (5,4)
angleBetween works on the directional vectors, to get the dirctional vector from point O to point A you have to substract them: O-A (or A.x-O.x, A.y-O.y)
do this as well for directional vector B-O and put those in the angle between:
angleBetween(A-O, B-O)

thank you for advise  janjacobsv :) 

The code calculates the angle between the vectors correctly.