.
1. Divide the angle A in two equal halves (bisect the angle A).
2. Then bisect any of the two halves.
3. You will get what you need.
On how to bisect the angle using a compass and a ruler, see the lesson
- HOW TO bisect an angle using a compass and a ruler
in this site.
For other construction problems in Geometry see the lesson
- OVERVIEW of lessons on Geometric construction problems using a compass and a ruler
in this site.
Also, you have this free of charge online textbook on Geometry
GEOMETRY - YOUR ONLINE TEXTBOOK
in this site.