The formula for circumference is 
or 
. We have a diameter for the semicircle, so let's use the diameter formula. 
which is either 
or C = 5.338 if we multiply pi in as 3.14. BUT this is only half a circle, so we only have half that distance around the outside. Therefore, the circumference of half this circle is 2.669 cm. But we need to add the 24 mm in. 24 mm in centimeters is 2.4 cm. So 2.669 + 2.4 = 5.069