Consider the circumcircle of triangle ABC. This circle will meet the
green line in at most two places.
Click for a Circumcircle script.
The angle ABC will be a maximum when the circle containing A,
C and B is tangent to the green line. The task then is to
construct a circle given A and C and the green line. This construction is
dealt with in Apollonius
problem.
Click here for the GSP script
This fact is also the principle underlying the construction used earlier.