In a geometric sequence the nth term is computed by:
[tex]a_n=a_1*r^{n-1}[/tex]
so in the given our nth term is 768, the 1st term is 6, the common ratio is 2 (we multiply 2 to get the next term).
So:
[tex]768=6*2^{n-1} \\ 768/6=2^{n-1} \\ 128=2^{n-1} \\ 2^7=2^{n-1} \\ 7=n-1 \\ 8=n[/tex]
Therefore 768 is the 8th term in the given sequence.