Plz the first and last term of an AP are 79 and -5 if the sum is 814. Find

Question

A. The number of terms
B. Common difference.

Answer 1

http://www.mathsisfun.com/algebra/sequences-sums-arithmetic.html

XN = 79+d(N-1) = -5
814 = N/2[ 2(79) + (N-1)d ]

814 = N/2 [ 79 -5 ]

N = 1628/74
N= 22
then
-5 = 79 +d(21)
d = -84/21 = - 4

Answer 2

where is the last answer