In the AP 5, 12, 19, …: which term is 166? is 300 a term?

The $n$th term of an AP is $a_n = a + (n - 1)d$. A number is a term only if $n$ comes out a whole number of at least $1$. The $n$th term from the end of a finite AP with last term $l$ is $l - (n - 1)d$.