Explanation:
The length and the width of the rectangle are LEGS of a RIGHT triangle
the 91 yards is the hypotenuse .....so you can use the Pythagorean Theorem for right triangles
hyp^2 = leg1^2 + leg2^2
91^2 = 35^2 + width^2
91^2 - 35^2 = width^2
7056 = width^2
then width = sqrt (7056) = 84 yards