Can someone plz show me how to do this. thx

Are you familiar with how to find the area of a rectangle?
A= base*height -- for rectangles

how about the area for a trapezoid?
well what happens when we cut the triangular section from one side and then fit it onto the other side, do we get a rectangle?

so finding the area of a trapezoid is essentially the same as finding the area of a rectangle.
In your case, t is the height,18 mi is your base and the area given is 216mi^2

"plug" those all into the rectangular area equation, divide the base from both sides, and you should be able to determine height

So to find area you have to use this rule : area= base•height and if you have area and one side you will have to divide to find another side: 216 : 18 =12

12 mi is missing length