How do I find a function $f(3n) = 3n$ such that it is different from the identity function? This is my first time posting. I'm sorry if I'm neglecting some good etiquette practices; I tried to read everything that's been sent my way, but I probably missed something anyway. Also, English is not my first language, so I'm relying on Google to translate math-specific terminology. If something isn't clear, please let me know!
I'm a Computer Science student at University, and I've been requested to find a function $f: \Bbb{N}\to\Bbb{N}$ such that $\forall n \in \Bbb{N}$,  $f(3n) = 3n \land f\neq \mathrm{id}_\Bbb {N}$. I absolutely cannot find a solution, as $f(x) = x$ (and, as such, $f(3n) = 3n$ too)  literally is the definition of identity function as far as I know... Am I missing something? Thanks in advance.
EDIT: Thanks a lot everyone!

Your function is from $\Bbb{N}$ to $\Bbb{N}$, so the equation
$$f(3n) =3n\tag1$$
means
\begin{align}
&f(3) = 3\\
&f(6) = 6\\
&f(9) = 9\\
&\cdots \cdots \cdots
\end{align}
so for natural numbers other than multiple of $3$, the function $f$ may have $any$ natural value and still satisfy the equation $(1)$.
So it is sufficient to assign e.g. $f(2) = 100$, and for all other natural numbers $f(n) = n$.
