To find the arc length we need to know the angle subtended by the arc.
From the diagram we notice that angle MNK is of 180°.
We also notice that:

Then the angle subtended by the arc MKL is 218°.
Now that we know this we need to remember that the arc length is given by:

in this case the radius is 3 ft (The radius is half the diameter) then we have that:

Therefore the arc length is 11.4 ft.