Answer
$ 17.7\ cm
.$
Work Step by Step
Using the Pythagorean Theorem, we have:
$${a}^2+{b}^2 = {c}^2.$$
Then
$$
b = \sqrt{c^2-a^2}=\sqrt{18.9^2-6.71^2}=17.668\ cm \approx 17.7\ cm
.$$
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