Hi.
I'm going to tell you the process, since I don't have a calculator on me. Don't wanna risk it, lol.
We can break this shake into 2 parts: a rectangle, and a triangle.
To separate them, just calculate the area of the rectangle with 4 km & 3.5 km
Since the triangle's area is half that of the rectangle, all we need to do is divide the area of the rectangle by 2 to get the area of the triangle (it's confusing when in words, eh?)
Once you find the area of both, add them together.
This should get you the area of the shape.
I hope this helped. Once again, please let me know if there are mistakes.