Explanation:
we need the length of the arc ?
the length of the arc fully around the circle (360°) is the circumference of the circle : 2×pi×r
and the arc length for 123° is then
2×pi×r×123/360
but we would need r to calculate that. you did not provide that.
the only other thing that comes to mind is : the arc angle is indeed the angle at the center of the circle and is 123°.
i don't know how else to help you, as i cannot think of anything else you might need there.