Perimeter is all of lengths of the sides added up. We know a square has four sides, all the same length. Therefore each side must be exactly ¼ of the perimiter.
So that's (16x-20)÷4
We can simplify that, by dividing both 16x and 20 by 4 to get (4x-5)
So the length of one side is (4x-5)ft
