Take into account that the length of arc can be calculate by the following formula:
s = r·θ
where r is the radius of the circle and θ is the angle (in radians) subtended by the length of the arc, related to the center of the circle.
In this case:
diameter = 9 ft
radius = diameter/2 = 9/2 ft = 4.5 ft
θ = 80°
to convert θ to radians, proceed as follow:
θ = 80°·π/180 = 1.40 radians
next, replace the values of r and θ to obtain s:
s = (4.5 ft)(1.40)
s = 6.283 ≈ 6.28 ft
Hence, the length of arc is 6.28 ft