Since AB is the diameter then arc ADB would be half of the circle.
so half of the circumference is 8pi.
full circumference would then be 8PI x 2 = 16pi.
Formula for circumference is 2*pi*radius.
Set that equal to 16pi and solve for the radius
16PI = 2 x PI x R
PI cancels out on each side
16 =2 x R
r = 16/2 = 8