The next term is double the previous term, so that the [tex]n[/tex]-th term is given recursively by
[tex]\begin{cases}a_1=1\\a_n=2a_{n-1}&\text{for }n>1\end{cases}[/tex]
This rule tells us that
[tex]a_2=2a_1[/tex]
[tex]a_3=2a_2=2^2a_1[/tex]
[tex]a_4=2a_3=2^3a_1[/tex]
and so on, with the explicit rule
[tex]a_n=2^{n-1}a_1=2^{n-1}[/tex]
for [tex]n\ge1[/tex].
If 512 is the [tex]k[/tex]-th term in the sequence, then
[tex]512=2^{k-1}\implies\log_2512=\log_22^9=\log_22^{k-1}\implies9=k-1\implies k=10[/tex]
