The initial term of the sequence is 15. The common difference is 3, so the n-th term (an) is
... an = 15 + 3(n-1)
For n=14, the 14th term of the sequence is
... a14 = 15 + 3(14 -1) = 54
The sum of terms is the average of the first and last, multiplied by the number of terms:
... sum = (15 + 54)/2·14 = 7·69 = 483
The appropriate answer choice is
... 54; 483
