I don't know what you want to do but if it is to eliminate a root from the denominator you need to multiply by the conjugate.
EG
So I haven't had alot of time with my surds, and I'm basically stuck on the principals, I sorta see where I'm going but then somehow its all wrong
I'll just show a few examples, then see if someone could explain it about (if you can =) )
2 /8 - 2 root 8
-2/18 + 6 - 6/2
-2 root 18 plus 6 - 6 root 2
/27 - /3 - 2/12
root 27 - root 3 - 2 root 12
Family has gone away the weekend, so no one I can really ask :S
Thanks you.