The diagnol of a rectangle is 20 inches. The width is 4 inches shorter than its length.
Your question, I assume, is to find the length and width of the rectangle?
The diagonal creates 2 right triangles.
L = length
L - 4 = width
By the Pythagorean theorem,
d^2 = L^2 + (L - 4)^2
20^2 = L^2 + (L - 4)^2
Solve for L, the length
width = L - 4