Answer:
123 + (456 + 789)
Explanation:
since this is an addition problem, the parenthesis don't matter, and they can be put wherever you want.
(123 + 456) + 789
123 + 456 + 789
(123 + 456 + 789)
123 + (456 + 789)
technically, any of these would work, but I think the kind of answer the problem is looking for is;
123 + (456 + 789)