Answer:
length = 17.3 m
Step-by-step explanation:
Rectangles have four right angles, so the diagonal and its sides will form a right triangle. This allows us to use the Pythagorean Theorem to find the length.
l^2 + 18^2 = 25^2
sqrt l^2 = sqrt (25^2 - 18^2), where l = 17.349 = 17.3 m