Keith Robinson bought two Siamese fighting fish, but when he got home, he found he only had one rectangular tank that was 12 in. long, 7 in. wide and 5 in. deep. Since the fish must be kept separated, he needed to insert a plastic divider in the diagonal of the tank. He already has a piece that is 5 in. in one dimension, but how long must it be to fit corner to corner in the tank?