Let me re answer that question: x = one piece
x + 7= other piece {one piece is 7 ft shorter than the other}
x + x + 7 = 31{the two pieces add up to the total 31 ft piece}
2x + 7 = 31 {combined like terms}
2x = 26 {subtracted 7 from each side}
x = 13ft {divided each side by 2}
x + 7= 20 ft {substituted 13, in for x, into x + 7}
the two peices are 13 ft and 20ft