The diameter is 6 feet long, so the radius is 6 / 2 = 3 feet long.
The perimeter of an entire circle will be
= 2 * pi * radius
= 2 * pi * 3
= 6 pi feet
Since this is a semi circle, we will need to half it:
6 pi / 2 = 3 pi feet
We will also need to calculate in the diameter, so the perimeter of the entire semi-circle is (3pi + 6) feet