Answer:
13y√ a
Explanation:
You can only add radicals if they have the same number inside of the radical, in which these two terms do.
Imagine If you had 6 Apples, and 7 more apples somewhere else, you would have 13 apples in total, right? This is the same principal, You are adding the amount of radical a together, in which you get 13y radical a.