B + N = 18 and 6B + 5N = 101. This is the system of equations you would use.
Well we set the perimeter to 120 feet.
This means that 2x+2y=120
Now we know the area of a rectangle is xy so we have to solve for both x and y in the perimeter equation.
2x=120-2y
x=60-y
2y=120-2x
y=60-x
Now we plug these values into our area equation A=xy to get:
A=(60-y)(60-x)
Answer: Third option.
Please, see the detailed solution in the attache file.
Thanks