The area of this figure is given y the product between the lateral area(red area) and the side length(blue length).
The lateral shape is a trapezoid. The area of a trapezoid is given by half of the sum of the bases times the height.

The smaller base of this trapezoid measures 2 mi, the bigger base measures 6 mi, and the height measures 3.5 mi. Using those values on the formula, we have the following area:

Then, multiplying by the blue length, we have the volume:

The volume of the figure is 28 mi³.