In a rectangle, opposite sides are equal and it forms two equal right triangles.

So we have .

To find:Length x which is hypotenuse c for both the right triangles.

Apply Pythagorean theorem:

              (Substitute 3 for a and 7 for b)

               (Evaluate powers: )

                     (Add: )

              (Take square root to each side)

                   (Cancel sqaure and root)

The length x of the diagonal of the rectangle is .