Solve the problem.
You have 84 feet of fencing to enclose a rectangular plot that borders on a river. If you do not fence the side along the river, find the length and width of the plot that will maximize the area.
◦ length: 42 feet, width: 21 feet
◦ length: 63 feet, width: 21 feet
◦ length: 21 feet, width: 21 feet
◦ length: 42 feet, width: 42 feet