13, 7, 1
a(1) = 13
a(2) = 7
d = a(n) - a(n - 1) = a(2) - a(1) = 7 - 13 = -6
a(n) = a(1) + d(n - 1)
a(16) = 13 - 6(16 - 1) = 13 - 90 = - 77
