Answers

2015-01-26T19:30:12+08:00
6y + 7y +(-4y)+(-11y)

= 6y + 7y - 4y - 11y 
= -2y

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. 

=19ac -21ab 

If I'm wrong, I'm sorry :'<

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