Answer
$$16 \ mi .$$
Work Step by Step
Since we have a right triangle with legs a, b and the hypotenuse c, then by the Pythagorean theorem, we have $$c^2=a^2+b^2.$$
Since $c=16.8 mi$ and $b=4.5 mi$, then $a$ is given by
$$ a=\sqrt{c^2-b^2}=\sqrt{16.8^2-4.5^2}\approx 16 \ mi .$$
