Answer
It is a factor of $f(x).$
Work Step by Step
The Remainder Theorem says that the remainder when a function $f(x)$ is divided by $(x-r)$ is $f(r)$.
Hence $f(-3)=2\cdot3^6-18\cdot3^4+3^2-9=0$, therefore $(x+3)$ is a factor of $f(x)$ by the factor theorem.
The factor theorem says that if $f(c)=0$, then $(x-c)$ is a factor of $f(x)$ and vice versa.