If you cut the rectangle (the wall) diagonally, you get two congruent triangles. The measurements of the sides of one the triangles is 8, 15 and x. There is a pythagorean triples 8, 15, 17 so x has to be 17. if you dont know the triple you can do a^2 + b^2 = c^2 8^2 + 15^2 = c^2 c = 17
