You are allowed to buy at most 9 books you have 3, find possible amount of books you can buy

Well, if I'm reading this question correctly, your answer is as follows:

1
2
3
4
5
6

I guess those would be your answers. Try asking this question again in a different way if you don't think that is right. The way you asked is sort of confusing.

Hope this helped! :)

Your answer basically lists from 1-6. So 1,2,3,4,5,6 I guess that's your answer

6(2x + 5y)6 (2x) = 12x6 (5y) = 30y12x + 30y