Answer:
a6 = -3072
Step-by-step explanation:
Put 6 where n is and do the arithmetic.
Since the form of your function looks like it is trying to describe a geometric sequence, we assume you actually mean ...
an = 3(-4)^(n-1)
So, for n=6, this has the value ...
a6 = 3(-4)^(6-1) = 3(-1024)
a6 = -3072
_____
Same deal if you really mean
an = -12n -1
a6 = -12(6) -1 = -73
