All 4 sides of a square are the same length.
If the perimeter of the square is 20-ft, then each side is 20-ft/4 = 5 ft.
If you increase the length of the square by 2-ft, then its new length is 5+2 = 7-ft.
If you decrease the width of the square by 1-ft, then its new width is 5-1 = 4-ft.
The area of the new figure is (length) x (width) = (7-ft x 4-ft) = 28 square feet.