Answer:

Explanation:
Think of this as an ordinary addition problem.
What must you add to 3a³ to get 1a³? Answer: add -2a³
3 + (-2) = 1
What must you add to -5b³ to get 1b³? Answer: add 6b³
-5 + 6 = 1
Then, the addition looks like this:

The numbers in the boxes are -2 and 6.