How to find the correct unit of measurement? i am struggling with this. what is the difference between ft³, ft and ft²? example:

the length of a rectangular play area is 40 ft. the width of the play area is 20 ft.

which is the correct unit of measurement to use when figuring out how much fence is needed to surround the play area?

a.
ft³

b.
ft

c.
ft²