Comment
The circumference of a circle is 2 pi * r
Any part of the circle's circumference can be labeled as
Arc length = (k / 2pi) * 2*pi * r
The 2pi s will cancel out and you are left with
Arc Length = k * r
In this case, k is in radians.
Givens
k = 4 pi / 7
r = 5 cm
Solve
Arc Length = 4*pi * 5 / 7 = 20 * pi / 7
Ans you are correct.