Hey there!
This problem is simply about combining like terms. Like terms in this scenario is anything with a similar or different coefficient and the same variable. Here, we see that everything is a like term, because they all have n as their variable.
When we have this situation, we can simply add or subtract the number like normal, and keep that n there. We have:
8n + 3n - n + 3n + 5n =
11n - n + 3n + 5n
Let's think about how to subtract that n. When we have 11n, we have n + n + n... 11 times. That's why multiplication is repeated additon. If we subtract one n, we take one n out to get 10n. We have:
10n + 3n + 5n = 13n + 5n =
18n
Hope this helps!