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.