x - 11 = y The distance between the acoustic guitar and sound hole is represented by y. The distance that the microphone should be from the acoustic guitar is represented by x. Since the guitar is supposed to be x distance from the microphone to the acoustic guitar but it is not, it is 11 inches less than what it should be. So 11 inches less than would be shown as - 11. You could also represent this with.... y + 11 = x x - y = 11