To solve this question we will use the following diagram as reference:
To answer the question we have to find the missing side of the right triangle, using the Pythagorean theorem we get:
![b=\sqrt[]{c^2-a^2}.](https://img.qammunity.org/2023/formulas/mathematics/high-school/wx0rqql2xa5wa1oe9hg0c480aluzx5ygd5.png)
Where a, and b are the legs of the triangle, and c the hypothenuse.
Substituting a=84, and c=91 we get:
![b=\sqrt[]{91^2-84^2}=35.](https://img.qammunity.org/2023/formulas/mathematics/high-school/3blqvu9tmyy0doxt82ovpcc6f4zfbbgjhl.png)
Answer: 35 ft.