Let k be a positive real number such that for the function
$$f ( x ) = \log _ { x } ( x - k )$$
we have $f ( 3 k ) = \frac { 2 } { 3 }$. What is k?\\
A) $\frac { 3 } { 8 }$\\
B) $\frac { 9 } { 8 }$\\
C) $\frac { 27 } { 8 }$\\
D) $\frac { 2 } { 9 }$\\
E) $\frac { 4 } { 9 }$