The 4 sides of a square are equal in length, so if the perimeter is 20ft that means each side would have to be 5ft, (20/4 = 5). If you increase the length by 2ft, one of the sides would now become 7ft (5+2 = 7). If you decrease the length of the other side by 1ft, the length would now be 4ft (5-1=4). Now to find the area, all you have to do is length * width. So you would do 7ft*4ft which is 28ft squared.
