D. 64x^9
This is because 4x^3 is the side length and you substitute it into l^3
and you get (4X^3)^3
(4X^3)^3 is the same as (4X^3) x (4X^3) x (4X^3)
1. (4X^3)^3
2. (4X^3) x (4X^3) x (4X^3)
3. (16X^6) x (4X^3)
(remember when multiplied, exponents get added to each other)
4. 64X^9
After multiplying all three of those you get 64X^9 D.
Hope this helps
