Using fzero to solve for extinction angle in MatLAB

Question

I am trying to find the best way to use Matlab to find the extinction angle for a single phase full wave controller. I have seen people create a graph that shows the intersection of two lines which they use as the angle. An example problem would be using the following parameters:

Vs = 100sin(2000pit); r = 20; L = 33mH; a = 40^o.

I understand the extinction angle is 291^o or so by using calculation methods but it seems it would be much easier to use the fzero method I have seen.

Thank you in advance.