Refer to the diagram shown below.
Note that
(i) the intersecting diagonals of the parallelogram bisect each other;
(ii) the supplementary angle of 60° is 120°.
Let
a = the length of the longer side
b = the length of the shorter side.
Apply the Law of Cosines to the lower triangle.
a² = 8² + 7² - 2(8)(7) cos(120)
= 64 + 49 + 56
= 169
a = 13 m
Answer: 13 m
