Both of these sides have to be equal, so we can say 4/3x -1/3 = x + 7, we can use lowest common denominators to combine like terms like so 1/3x = 22/3, so we can solve to get x = 22. We know the perimeter is the sum of the lengths of each side, an octagon has 8 sides, each side is 22 long, so 8*22 = 176 is the perimeter.
