I'm trying to find if this function is odd or even :

$f(x) = \log(3x + \sqrt{9x^2 + 1})$

I know that it's an odd one because if I try **$f(x) + f(-x) = 0$** it shows that it's odd. But I want to know how to figure problems like this?
I used this method <http://www.purplemath.com/modules/fcnnot3.htm> ( plug -x to check ) but its not working here.