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. 

This is the solution (be aware that your figure is misleading):
$$ A = (150 - 100, 50 - 100) = (50, -50) $$
$$ B = (180 - 100, 100 - 100) = (80, 0) $$
$$ \cos\Theta = \dfrac{A \cdot B}{|A||B|} = \dfrac{400}{50\sqrt{2} \times 80} = \dfrac{1}{\sqrt{2}} $$
$$ \Rightarrow \Theta = \dfrac\pi4 $$
