The key here is to remember how to get rid of the parentheses.
The 'minus sign' ( - ) before the parentheses says that in order to
get rid of them, you have to change the sign of each term inside.
-3a - (5 - 6a) is the same as
-3a -5 + 6a
Add up the (-3a) and the (+6a) and you have +3a .
So the whole thing becomes 3a - 5 .
