Can someone tell me why we need mask it's due in a hour (btw this is a fake question I just made something up for free points so you can say anything)

Aconstruction worker needs to put a rectangular window in the side of a building he knows from measuring that the top and bottom of the window have a width of 8bfeet and the sides have a length of 15 feet he also measured one diagonal to be 17 feet what is the length of the other diagonal