He needs to fence the entire perimeter, so all you have to do is find the perimeter. P= S+S+S+S, which is 125, 200, 125, and 200. Add them together and you get 650, so he needs 650 feet of fence.
The first equation will turn into -18x -6y = -18, and the second equation will turn into 14x + 6y = 18. The y terms will cancel out when you add the equations together.
