6ab-(12ab-4ab) = 6ab+ -1(12ab-4ab) there is an invisible 1 between the minus sign and the bracket.
= 6ab+(-1)*12ab+(-1)*-4ab
= 6ab-12ab+4ab
= you can do the rest.
This is the full explanation but you can just remember that if there is a minus sign outside the bracket like this one all the signs inside the bracket get changed when the brackets are removed.
For example
-(-a+6-2g) would become a-6+2g
I hope that you can learn from this.
+118654 
