Sequence and series

I want to ask that in this question i am given 3n+8 , 7n+15 is this two expression 3n+18 and 7n+15 it is general formula of any arithmetic progression that means when i put value of n in the expression 3n+8 i get ap? I solved it but i want to visualise question

@godisgreat yes you are right for different values of n we get different terms of ap