To solve part a, we going to need the circumference of a circle formula
Being r the radius of the circle
So for our case:
Answer a: 8pi or 25.1
Now for the part b, we got
So let's set out this equation
In this case, beta is the length of AB and pi/3 is the conversion to radians of 60
Solving we get
Answer b: 4pi/3 or 4.1889