Chapter 8 - Section 8.2 - Solving Quadratic Equations by the Quadratic Formula - Exercise Set - Page 493: 57a

$$x=50 \sqrt2$$

For part a, we are asked to understand the problem. Here, we are given that the diagonal has length 100 meters. Each unknown side we will call x, since they are the same length. This gives: $$x^2+x^2=100^2$$ $$x^2=5000$$ $$x=50 \sqrt2$$