Jon S. answered 09/24/21
Patient and Knowledgeable Math and English Tutor
The diagonal (x) would be the hypotenuse of the right triangle and the length (21) and width (7) would be the legs.
By the Pythagorean theorem: 21^2 + 7^2 = x^2
x^2 = 490
x^2 = 49 * 10
x = 7 * sqrt(10)