I'm sure you already know that cubic yards are a measurement of the volume and so this question is just asking for the length times width times height.
However, you have an area (40 acres) and a depth, so you don't need length and width. All you need to do is convert 20 feet to yards and to convert 40 acres to square yards, then multiply.
There are 4840 square yards in each acre.
Hope this helps,
Stephen La Rocque.