Answer
$59.8 \ km$
Work Step by Step
Using the Pythagorean Theorem, we have:
$${a}^2+{b}^2 = {c}^2.$$
Then
$$
b = \sqrt{c^2-a^2}=\sqrt{61^2-12^2}=59.808 \ km \approx 59.8 \ km
.$$
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