Answer
$f(n)=5n^{\log_32}-4$
Work Step by Step
We are given
$$n=3^k$$
$$f(n)=2f(n/3)+4$$
$$f(1)=1$$
Applying to Theorem 1 we have:
$f(n)=C_1n^{\log_32}+C_2$
where $C_1=f(1)+\frac{4}{2-1}=5$
$C_2=-\frac{4}{2-1}=-4$
Hence $f(n)=5n^{\log_32}-4$