According to the angle bisector theorem, if a point is on the bisector of an angle, then it is equidistant from the sides of the angle. You can tell this angle is a bisector because the measurement of the top two angles are congruent. When the triangle is bisected, that means it is cut in half. The line at the bottom was cut in half too. Because one side of the line (CD) is 6, the other side of the line (DB) is 6 too.
