We can calculate the area of the trapezoid by multiplying the height and the average of the top and bottom parallel sides.
The height is always measured perpendicularly to the parallel sides, as is shown in the picture.
Then, we can calculate the area as:
![\begin{gathered} A=((a+b)/(2))\cdot h \\ A=((5+9)/(2))\cdot3 \\ A=(14)/(2)\cdot3 \\ A=7\cdot3 \\ A=21\operatorname{km}^2 \end{gathered}]()
Answer: the area is 21 km^2.