A boardwalk that is x feet wide is built around a rectangular pond. The combined area of the pond and the boardwalk is 4x^2+140x+1200 square feet. What are the dimensions of the pool?
a. 100 ft by 60 ft c. 55 ft by 70 ft
b. 30 ft by 40 ft d. 15 ft by 80 ft
...?