So since in a rectangle, there are two pairs of congruent sides, you can double 15 and take it out of 46, which leaves you with 16, and when you divide that by to, you get 8.
So you have two sides with length 15 feet, and two sides with length 8 feet.
And if I understood the question correctly and you're looking for the diagonal of the rectangle, then it's 17. (because 8, 15, 17 is a common right triangle)
