Answer
$13\,m$
Work Step by Step
As can be seen from the figure, the two diagonals intersect each other.
Using the law of Cosines:
$$b^2 = 7^2+8^2 - 2 \times 7 \times 8 \times \cos{120^\circ} \\ = 169$$
$$\therefore\boxed {b = 13 \, m}$$
Trigonometry 7th Edition
by McKeague, Charles P.; Turner, Mark D.
Published by
Cengage Learning
ISBN 10:
1111826854
ISBN 13:
978-1-11182-685-7
Chapter 7 - Section 7.2 - The Law of Cosines - 7.2 Problem Set - Page 378: 29
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