6y + 7y +(-4y)+(-11y)
= 6y + 7y - 4y - 11y
21ac + (-2ac) + (-8ab) + (-13ab) ** I put the ones with common variables together
= 21ac - 2ac | - 8ab - 13ab
**since you cannot subtract nor add -2ac with -8ab , I separated them. Since they do not have the same variables.
If I'm wrong, I'm sorry :'<