You need 3a^3 to equal a^3, so you would need at subtract 2a^3.
Since the problem is addition, add - 2a^3:
3a^3 + -2a^3 = 3a^3 - 2a^3 = a^3
Then you have -5b^3 and you need it to equal b^^3, so you would need to add 6b^3:
-5b^3 + 6b^3 = b^3
First box would be -2
Second box would be 6
