Hi, I've got a question here and I've got to show that a function is not surjective.

To do this I've got to find a value that the function cannot equal, but I'm not sure how to do this, is trail and error the right way or is there a more formal approach?

Here's the actual function:

$\displaystyle f : \mathbb{R}$ \$\displaystyle \frac{1}{3} \to \mathbb{R}$ given by $\displaystyle f(x) = \frac{5x+1}{3x-1}$

Thanks in advance