Where the image has the opposite orientation as the preimage

  7.5 meters

Step-by-step 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

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.