I'm having issue solving this problem

If they were said that xi are non-negative integers I would know how to solve it. It would be n = 32, k = 6, and then (32+6-1) chooses (6-1). But I'm not sure how to apply any of these constraints. My professor didn't go into detail about it and only gave us one example in class. I would really appreciate it if you could provide me with some explanation too! Thank you!