These are not like terms so we cannot add them together as they stand. (Like radical terms have the same kind of roots with the same radicands. "Radicand" is the name for the expression inside a radical.) Your terms are the same kind of root, cube root, but the radicands are different.
We can, however, simplify the cube roots since they each have a perfect cube factor:
The cube roots of the perfect cubes simplify:
which simplifies to:
With the roots simplified, we can see that the terms are now like terms. We can now add them together. And exactly like 3x + 4x = 7x:
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