The Pythagorean theorem says that
a^2 + b^2 = c^2
a and b are both legs of the right triangle and c is the hypotenuse.
The legs each measure 7 miles so…
7^2 + 7^2 = c^2
49 + 49 = c^2
98 = c^2
You then have to take the square root of 98 as c is still squared.
The square root of 98 is about 9.9
c is about equal to 9.9