Answer
$$19,600\ ft .$$
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 $c=42,600 ft$ and $a=37,800 ft$, then $a$ is given by
$$ b=\sqrt{c^2-a^2}=\sqrt{42,600^2-37,800^2}=19,600\ ft .$$
