The formula of the area of the trapezoid is:
[tex]\displaystyle{ A_{trapezoid}= \frac{1}{2} h(b_1+b_2)[/tex],
where h is the height, and b_1 and b_2 are the two bases of the trapezoid.
In our problem h=3km, and the bases are 5km and 7 km. Substituting these values in the formula, we find:
[tex]\displaystyle{ A_{trapezoid}= \frac{1}{2} h(b_1+b_2)= \frac{1}{2} \cdot 3\cdot (5+7)=\frac{1}{2} \cdot 3\cdot 12=3\cdot6=18[/tex]. (km)
Answer: B. 18
