The figure consists of a semi-circle and a triangle.
Hence,

Given a triangle with base b and perpendicular height, h, then the area, A, of the triangle is given by

In this case,
b = 3km, and h = 4km
therefore,
![\text{ area of the triangle = }(3*4)/(2)=3*2=6\operatorname{km}^2]()
Given a semi-circle with diameter, d, the area, say S, of the semi-circle, is given by


We must have that,

In our case,
d = 4km
therefore,
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