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-21a - 42b + 14a - 9b simplified

combining like terms

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2 Answers

Combine the A's and the B's using their signs as well.  so  -21a  + 14a  = -7a     just add the -21 and +14

do the same for the B's  -42b - 9b  = -51b    therefore your answer is  -7a -51b


Approach these problems the same way you would if you were adding apples and bananas. 
If you had:
                  -21 apples -42 bananas +14 apples -9 bananas

how would you do it?  You'd separate the apples and bananas, of course.  -21a +14a = -7a; -42b -9b = -51b. 
Look for common terms and combine as appropriate.