Answer:
7.5 meters
Explanation:
As with many quadrilaterals, pairs of sides have the same length, so the perimeter is twice the sum of two of the sides.
In a kite, generally, opposite sides have different lengths, so the perimeter is twice the sum of the lengths of opposite sides. That is
51 m = 2(18m + side opposite)
15 m = 2 × (side opposite)
7.5 m = side opposite
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Comment on side lengths
In a rectangle or parallelogram, the perimeter is twice the sum of adjacent sides. A kite is different in that adjacent sides may be the same length. If the kite is not a rhombus, opposite sides are always different lengths.