Take into account that the perimeter of a circle is its circumference, which is given by:
C = 2πr
where r is the radius of the circle. For a semicircle, the perimeter is:
C' = 2πr/2 = πr
the radius of the given semicircle is:
r = d/2 = 20 cm/2 = 10 cm
replace the previous value of r into the formula for C':
C' = π(10 cm) = 10 π cm this is the perimeter of the given figure