How to calculate an angle from three points? [closed]

If you mean the angle that P1 is the vertex of then using the Law of Cosines should work:

arccos((P₁₂²
+ P₁₃² – P₂₃²) / (2 *
P₁₂ * P₁₃))

where P₁₂ is the length of the segment from P1 to P2, calculated by

sqrt((P1_x –
P2_x)² +
(P1_y –
P2_y)²)