You just have to remember these:<br>
a+b=!a\*!b<br>
a\*b=!a+!b<br>
plus of course !(a+a)=!a, !(a+0)=!a, !(a\*a)=!a, !(a\*1)=!a<br>

So for example 3 input NOR = !(a+b+c)=!((a+b)+c)=!((!a*!b)+c)=!(!(!a*!b)\*!c)<br>
Hope this helps