getting the inner corner angle I have four points that make concave quad:

now I wanna get the inner angle of the (b) corner in degrees.
note: the inner angle is greater than 180 degree.

Let $\alpha$ be the angle from $bc$ to $ba$ then
$$\tan \alpha=\frac{m_2-m_1}{1+m_1m_2}$$
where $m_1$ is the inclination of $bc$ and $m_2$ is the inclination of $ba$.
$m_1=\frac{10-9}{18-11}=\frac{1}{7}$
$m_2=\frac{9-11}{11-3}=-\frac{1}{4}$
you want to find $360^o-\alpha$.
Edit:
With a calculator:
$\alpha=202.17^o$
