List the values at which the function

$\displaystyle

f(x) = \frac{1}{x^2+9}

$

Is not defined.

This question does not have an answer in my text book so I cannot deduce where I have gone wrong, though actually I haven't gotten anywhere...just staring at it and repeating it to myself. I've tried factorising it out, but that doesn't work see (x+3)(x-3)..... $\displaystyle {x^2}$ must be a positive number right?