Equilateral triangle ABC is inscribed in circle X. What is the measure of arc CB?

Explanation:

An equilateral triangle has three sides of the same length, which in turn means that the curved arc pieces AB, BC and AC are the same length (but longer than each side of the triangle).

Furthermore, it leads to the fact that we split a 360 degree circle into three equal portions of 360/3 = 120 degrees each.

Imagine we drew in the line segments AX, BX, and CX. This splits the circle into 3 equal pie slices. Central angle BXC is 120 degrees because of the earlier calculation done before.