A park is shaped like a rectangle with a length 12 times its width w. What is a simplified expression for the distance between opposite corners of the park?

The opposite corners would be the Pythagorean theorem, because a rectangle has 90 degree corners, which would then be 12^2 + w^2 = d^2, where d is the distance between the opposite corners of the park, w is the width and 12 is the length!