I don't see a figure, so I'll mention two possibilities. There are more possibilities. Since the arc is named arc AC, points A and C must be on the circle. Since I don't see the figure, I don;t know where point B is.
If point B is on the circle, then angle ABC is an inscribed angle. The measure of an inscribed angle is half the measure of the intercepted arc, so m<ABC = 156/2 = 78.
If point B is the center of the circle, then angle ABC is a central angle. The measure of a central angle is the same as the measure of the intercepted arc, so m<ABC = 156.
Do you have one of the two situations above?
