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a+(2a+1)=13
We move all terms to the left:
a+(2a+1)-(13)=0
We get rid of parentheses
a+2a+1-13=0
We add all the numbers together, and all the variables
3a-12=0
We move all terms containing a to the left, all other terms to the right
3a=12
a=12/3
a=4
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