Calculate the angle from the given points coordinates. I'm trying to figure out the way to calculate the a angle value from given coordinates of three points as showed on the illustration below:

I know how to calculate the a angle from the triangle's base length and its height, but in this case I'm stucked. I'll appreciate any help.
EDIT
The a value should be expressed in degrees. Also, the points coordinates can vary. 

Drop a perpendicular from $x_1y_1$ to get a right triangle with the adjacent and opposite of $a$ both equal to $50$. Hence $a=\pi/4$ (because $\tan a = {O\over A} = {50\over 50} = 1$).
EDIT: Note $\pi/4$ radians equals $45$ degrees. The result here is invariant under translation (it stays the same when the points move).
