Answer
$1770\ m$
Work Step by Step
Using the Pythagorean Theorem, we have:
$${a}^2+{b}^2 = {c}^2.$$
Then
$$
a= \sqrt{c^2-b^2}= \sqrt{3960^2-3540^2}=1774.82\ m \approx 1770\ m
.$$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.