4.3 kilometres
1. Find the diameter by using the formula of the perimeter of the semicircle:
P=1/2 pi d(diameter)
5.14=1/2×3.14×d
5.14=1.57d [divide all sides by 1.57]
1.57. 1.57d
rounded off it gives you the diameter as 3.3
2. Then find the area using the formula
A=1/2 pi r²
=1/2×3.14×1.65(half of 1.57)×1.65
this gives you the answer as 4.3 kilometres rounded off
Hope this is understandable