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:
$$b^2 = 7^2+8^2 - 2 \times 7 \times 8 \times \cos{120^\circ} \\ = 169$$
$$\therefore\boxed {b = 13 \, m}$$
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.