Answer:
diagonal measure(s)
Explanation:
Opposite sides are the same length for any parallelogram, as suggested by the line drawing at the upper right of your figure. The figure will be a parallelogram only if the angles are right angles.
You can check to see if the angles are right angles in two ways:
1. see if the diagonal satisfies the Pythagorean theorem. The sides and diagonal will satisfy the Pythagorean theorem only if the corner angle is a right angle. A parallelogram with a right angle is a rectangle.
d² = 30² +72² = 900 +5184 = 6084
d = 78
2. see if the diagonals are the same length. Diagonals of a parallelogram are the same length if and only if the figure is a rectangle.
One or both diagonal measures will tell you if the stakes mark a rectangle.