Area of the deck=128 square feet
Area of the section which has been added to the deck=s²
s² =Represents a Section which is in the shape of Square.
Because area of Square =Side(s) × Side(s)
→Deck is a wooden structure, approximately in the shape of Rectangle.
Also, considering , 's' the side of square a not a large Quantity, Quite smaller with respect to area of Deck.