Since the number of nail boxes is 6 less than the number of yards of wood, the right expression to use is x-6. (x being the number of yards of wood) Now let us calculate the number of nail boxes she bought. Cost of x yards of wood at $27 per yard = 27x. Cost of x-6 boxes of nails at $4 per box = 4(x-6) = 4x - 24 Therefore 27x +4x - 24 = 193 31x -24 = 193 31x = 193 + 24 31x = 217 x = 217/31 = 7 So she bought 7 yards of wood and 7-6 boxes of nail. That is, she bought only one box of nail.