Determine all of the critical numbers of the given function.

I'm like 90% sure that means solve for x, check if what you got for x actually works, and that/those are your critical number(s).

Anyways, here's the problem:

Any ideas anyone? I tried solving for x:

then

then squared both sides

then moved some stuff around

and now I'm stuck there, not even knowing if I'm supposed to be doing this.

HALP