Volume is how many units an object can hold inside of it. I am not 100% sure that this would be the correct way to solve the problem but here is my suggestion:
Since volume is how many units an object can hold inside of itself, and we know that this rectangular prism can hold 756 wooden blocks with edge lengths of 1/3 ft., I would suggest first finding the volume of those little wooden blocks and then multiplying that answer by 756. Since each wooden block has 1/3 ft. as its edge measurement, multiply 1/3 x 1/3 x 1/3. This will give an answer of 1/27, or 0.037 repeating if you use a decimal. Now multiply that by 756. When 1/27 is multiplied by 756, the answer is 28.
So your answer should be 28 ft.^3
***** Note: Like I said in the beginning, I am not 100% sure that that is the correct way to solve the problem, but that is how I would try to solve it. ******