Answer:
The missing measurement is 9 miles
Step-by-step explanation:
we know that
The Area of a parallelogram is equal to

where
b is the length of any base
h is the corresponding altitude
The altitude (or height) of a parallelogram is the perpendicular distance from the base to the opposite side (which may have to be extended).
In the figure, the altitude corresponding to the base is 4 miles
substitute

Solve for b
Divide by 4 both sides

therefore
The missing measurement is 9 miles