i am not sure how to do this so please help.
This is a simple case of the Pythagorean theorem. The diagonal represents the hypotenuse.
Long side is 84 meters. So finding the width is trivial if you plug in the numbers in the formula.
Remember the Pythagorean theorem states that
Hypotenuse^2 = adjacent^2 + opposite^2
91^2 = 84^2 + w^2 (we call width w)
w = 35