A GP has the form a, ar, ar^2, ... where a is the initial term and r is the common ratio between terms, so ar^6 is the 7th term, and if a sub 0 is the first term=ar^0, a sub 10 is the 11th term=ar^10 and a sub 13=ar^13. So ar^10=9 and ar^13=-72. Divide a[13] (a sub 13) by a[10]=-72/9=-8=r^3, so r=-2. We can also find a=9/r^10=9/1024. Now we have enough to find ar^6=(9/1024)*64=9/16.