because you are measuring the perimeter of a rectangle. the rectangle has four sides, 2 for width and 2 for length. 75 for width, times 2 makes 150. plus 200 from the 2 lengths combined makes 350. now, the instructions state that the fencing will start at one corner or the house and end at the other corner, thus no fencing is required for the back side of the house which is 40. then you subtract 40 from 350.