Answer: 
4 cm 
Explanation: 
Radius = 4 
Using the formula: (This is just for radius): 
(8π-12√³)
Use the radius to work out circumference: 
C==2 π r==2··π··4=≈25.13 cm 
Divide the circumference by 6 because the AB curve is 1/6 of the whole circumference: 
25.13 divided by 6 = 4.188333333333333
(Corrected to 1 sig fig) 4 cm  
Side note: 
It is not 4 (with the pi sign at the end),
this is because the numbers do not go on infinitely.
Hope this helps! 
Please give Brainliest!