A billboard designer has decided that a sign should have 3-ft margins at the top and bottom and 1-ft margins on the left and right sides. Furthermore, the billboard should have a total area of 1200 ft(including the margins). If denotes the left-right width (in feet) of the billboard, find the value of a that maximizes the area of the printed region of the billboard. O 20 O 10 O 30 O 40 O 50