Answer
$8 \pi \ ft$
Work Step by Step
Let $l$ be the length of an arc:
$l=\dfrac{\text{Measure of the arc}}{360^{\circ}} \times 2 \pi r ~~~(a)$
or, $l=\dfrac{\text{Measure of the arc}}{360^{\circ}} \times \pi d~~(b)$
Here, $r$ represents radius and $d$ is diameter.
$180^{\circ}-60^{\circ}=120^{\circ}$ and $d=24 \ ft$
Thus, equation (a) becomes:
$l=\dfrac{120^{\circ}}{360^{\circ}} \times \pi (24)=8 \pi \ ft$