3a^3 is the answer
Hope that helps
Simplifying makes a algebric expression easily understandable and solvable.

is already in simplest form.
But if you meant to say

, we would combine the first two terms.
Adding/subtracting like terms is the same as adding/subtracting whole numbers.

Therefore:

Which gives us:
5/1, 15/3, and 10/2 are all valid answers to that question.