A^2 + B^2 = C^2 is known as the Pythagorean theorem. This theorem explains the proportionality of the 3 sides of a right triangle (a triangle with 1 corner angle = 90 degrees). If you know the length of 2 sides (in his example, the wall beams) you can find out the length of the third (in his example, this would be the supporting strut spanning the beams that meet at a 90 degree angle). If their example is explaining a beam that spans the room from 1 corner to the other, you still use this formula as a rectangle is 2 right triangles that meet along their hypotenuse (the longest leg of a right triangle, or the length you are solving for in this problem). The 2 known sides are the length/width of the room, and you solve for the 3rd side, your diagonal beam