16b. Here is the formula for nth term of an arithmetic sequence that has same distance between terms. Suppose a1 means first term, d is the difference between neighboring terms, and n is the number of the term you seek.

terms: a1, a1+1d, a1+2d, a1+3d, ...

nth term: a1 + (n-1) * d

In this problem a1 is 12, n is 15, and d is the common difference you see between any two terms, such as 19 - 12.