Width of the rectangular window = 5 feet
Length of the rectangular window = 12 feet.
Length of one of the diagonals is = 13 feet
As it is a rectangle, both the diagonals are equal, so other one will also be 13 feet. Or we can solve it by using Pythagoras theorem.
Let the supposed diagonal be = d



d=13
Hence, the length of the diagonal will be 13 feet.