the answer:
the calculus of the circumference depends on what is the value of central angle, so we can take for example 120° the value of the central angle.
since the length of arc ab is 28.61, and the central angle is 120°, we can find the circumference simply by using the following formula:
ARC = Radius x Angle, so Radius = ARC / Angle = 28 .61 / 120° = 0.23 unit
the circumference of a circle is C = 2 x radius x Pie
C = 2 x radius x Pie = 2 x 0.23 x 3.14 = 1.49 unit