Add or subtract the coefficient or the number before each radical, then copy the radical for similar radicals. Note that if you don't see a coefficient, assume there is a number "1".
The rule is, you can only subtract or add radical expression with same index and radicand.
Ex: 4∛21 + ∛21 = 5 ∛21
Note that if there is no coefficient before the radical, assume there is number 1