Question

A fence is ito be nstalled around a semicircle

Answer 1

If the lenght of a circle circumference is given by:
$c = 2\pi \times r$

So the lenght of a semicircle circumference is given by the half of it:
$c \: (semicircle) = (2\pi \times r) \div 2 \\ c \: (semicircle) =\pi \times r$

Radius (R) = 12 ft
$c \: (semicircle) =12\pi \: ft$

Answer = 12pi ft of fence.