The common difference is 19-7 = 31-19 = 12, so this is an arithmetic sequence. The initial term (when n=1) is 7.
The explicit formula for the terms of an arithmetic sequence is an = a1 + d(n-1) where "a1" is the first term and "d" is the common difference. The formula that matches a1=7 and d=12 is B. a(n) = 7+(n-1)·12
