If we're to find the exact value,we must not change 兀 into any other numbers to keep it's value.
The formula of volume of sphere:

Thus the exact volume would be:
4/3兀×6.5^3
=4/3兀×274.625
=2197/6兀 ft. cube
We also must not take demical places as that would affect the exact value as well.
Thus the answer is:2197/6兀 ft. cube
Hope it helps!