3∛(2a)-6∛(2a)
Those are LIKE TERMS:
That's just like
3x-6x are LIKE terms.
It's just that instead of a simple x you have a complicated ∛(2a)
But that doesn't make the problem any more complicated.
Just as
the answer to 3x-6x is -3x, because they are LIKE TERMS, and
you just combine coefficients 3-6 and get -3, and then put x after that,
the answer to 3∛(2a)-6∛(2a) is -3∛(2a), because they are LIKE TERMS, and
you just combine coefficients 3-6 and get -3, and then put ∛(2a) after that.
Therefore 3∛(2a)-6∛(2a) is just -3∛(2a)
Edwin