The common difference is multiply f(n-1) by -6 and (f)1=-4
mutiply -144 by -6, then 864 by -6, then -5184 by -6. then 31104 by -6 to get -186624
so we have -4, 24, -144, 864, -5184, 31104, -186624 as the seven terms in the geometric sequence, now we just need to add these together and getting the final answer of -159,964
