Mrs. Thomas is building a deck for her mountain cabin, where she plans to host summer cookouts for family and friends. Her cabin covers a rectangular area that is 40 feet by 30 feet. The deck will go around two sides of the cabin and be the same width on both sides. The total area of the cabin and deck in square feet can be modeled by the expression (30 x)(40 x), where x is the width of the deck in feet. This expression can also be written in the form x^2 70x 1,200. What does the quantity x^2 70x represent in the expression?