This sequence is arithmetic because we add -2 to each term to get the next term. So d = -2 is the common difference.
The starting term is a1 = 21
Therefore the nth term formula is
an = a1 + d(n-1)
an = 21 + (-2)(n-1)
an = 21 - 2n + 2
an = -2n + 23 which is the final answer
note how plugging in n = 1 leads to
an = -2n + 23
a1 = -2*1 + 23
a1 = -2 + 23
a1 = 21 which is the proper first term we want
So that helps partially confirm the answer. I'll let you check the other terms.
