An acre is a unit of area, not a unit of length. You can't convert that.
Actually, an acer is a species of tree - like the maple.
The answer is 27 Cubic meters. (3 meters long X 3 meters wide X 3 meters high = 27 cubic meters.)
An acer is a kind of tree so I am not sure about the number of square metres in 4000 acers. If the question is about acres, the answer is 16,187,426 sq metres approx.
how much fencing will be needed to fence a property 6.5 meters long and 3 meters wide?
2 meters x 3 meters = 6 square meters floor space.
9 feet