The 2 {} you have are called sets. The union is what the 2 sets have in all.
You can think of this as you have green blue yellow cards. and your friend have green orange red cards. What colors do you have if you combine your cards.
Now that's just an example, in the question's case is what numbers does the 2 sets have in total.
Therefore the union of these 2 sets are {9,10,11,13}
