What "along x-axis means in polar coordinates" and how to find the limit of $f(x,y)= \frac{3(x-y)^2}{9(x^2+xy+y^2)}$ in polar coordinates?

Question

Use polar coordinates to evaluate the limit of a function $f(x,y)=\frac{3(x-y)^2}{9(x^2+xy+y^2)}$ along the $x$-axis.

I know that to transform it into polar coordinates I have to assume $x=r\cos\theta$ and $y=r\sin\theta$. And along $x$-axis means $y=0$. But what does along the $x$-axis mean in polar coordinates? How to proceed for this problem?.