Which property of algebra should be first applied to 6a - 14b + 3a - 6 + 21b - 18a in order to determine whether it is equivalent to -9a + 7b - 6?

In order to determine whether the expression 6a - 14b + 3a - 6 + 21b - 18a is equivalent to -9a + 7b - 6, we should first apply the commutative property of addition.