This is a geometric sequence. To find the common ratio, divide the second term by the first term.
-16/8 = -2
an = a1 r^(n-1)
n = term to find = 13
a1 = first term = 8
r = common ratio = -2
a13 = 8 -2^(13 - 1)
a13 = 8 -2^12
a13 = 8 4096
a13 = 32768 <===
