I have 2 points, A + B, with vectors from the origin a and b. The vector from A to B is
c = b - a
a and b are defined in polar coordinates with a $ = (r_a,\theta_a) $, b $ = (r_b,\theta_b ) $
I want to know how to define the angle c makes with the horizontal in terms of $\theta_a $ and/or $\theta_b$.