Arectangular closet had a perimeter of 20 feet and a width of 3 feet. find the diagonal.

Use the Pythagorean theorem when you draw the diagonal in the rectangle. When you draw the diagonal you create 2 right triangles and there for you can use a^2+b^2=c^2
20^2+3^2=c^2
400+9=c^2
409=c^2
Take the square root of 409 to get 20.224