Answer:
15.65 miles
Explanation:
The diagonal of a rectangle is basically a divider for two triangles whose areas are the same inside the rectangle.
Using Pythagorean Theorem to find the hypotenuse (the longest side aka the diagonal) - this can be written in two ways:

or
c =

Let's use
.
"A" represents one side length/leg, "B" represents the other side length/leg, and "C" represents the hypotenuse/the longest side (aka the diagonal in this case).
We are given two side lengths (legs):
A = 14 miles long
B = 7 miles wide
Plug it into the formula;
c =

c =

c =

c =

C = 15.6524758425
Let's round to the nearest hundredth:
C = 15.65 miles is the hypotenuse.