To find this you need to observe that the function does not make sense when the denominator is zero, otherwise we have division by zero, which is definitely not allowed.
So, for what value of x is the denominator zero? Well if the denominator is zero then we have;
Can you rearrange this to get x on it's own?
Let me know if you get stuck.
EDIT: Sorry, I was too hastey with my reply. I disregarded the fact that you need the square root to be of a positive number (or zero). Observe that this happens when and so