Oliver, don't let letters in math confuse you. They are simply placeholders, instead of saying 3 + ____ = 9, we say 3 + t = 9, what is t?
We can combine the same letters, but not with numbers. If I said Sophie has 8 books, you'd say Joanna has 8 + 3 = 11 books. Joanna instead has n + 3 books. That's your final answer.