Find angle of an arc in the circle using 3 coordinates

Question

I want to find angle of semicircle. I have 3 coordinates (center_a,center_b) , (pivot_a,pivot_b) and (last_point_a, last_point_b).

From triangle , i can find angle using equation

using cosine inverse ,we get angle. But on circle, angle can be 0 to 360. Can anyone help me to find angle of arc?

Thanks