It's an angle bisector and it was found using a compass
Step-by-step explanation:
First you use the compass to make a marking in the angle
Then you move the compass to the D and make another marking after that you move the compass to E (Make sure to use the same compass setting) and make a marking that intersects with the marking of D which creates F. Last you use a straight edge to make the angle bisector of ABC. (Sorry if you don't understand this it was difficult for me to explain)
