Okay this is a little poorly worded but I think what they're trying to say is each vase has the same number of red and yellow roses.
Colise has 10 red roses and 5 yellow roses in each of two vases. To get the total we simply add 10 and 5 to get 15 and then multiply by the number of vases, and we get 30.
Now we know Michelle has a total of 30 roses. We need to figure out how many of them are yellow. We already know how many are red so we'll use that. To find the number of red roses we just multiply 6 by the number of vases to get 18. We now know that he has 30 roses and that 18 of them are red, that makes 12 of them yellow. Then we distribute the 12 yellow roses into the 3 vases by dividing 12 by 3 and we finally get the answer, 4 yellow roses in each vase.
To check your math just add 4 and 6 to get 10 and then multiply by 3 to get 30 total roses, the same amount as Colise.