Hello!
Here we have s=r*thetha and s is arc length which is what you want here. Also r is radius and theta is in radians only! So given your values we have
s=3.5/2*(20revs)*2pi
(because 2pi is one rev in radians)
s=1.75*40pi = 70*pi for an exact answer. Or 70*pi= 220ft
Hope this helps! Any questions please ask!!
Thank you!!