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2014-11-07T23:18:47+02:00
3a+b=12
3(a-b)=12⇒a-b=4


3a+b=12
a-b=4
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3a+b+a-b=12+4
4a=16
a=16:4
a=4

b=a-4=4-4=0
b=0