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3(2r+1)=21
We move all terms to the left:
3(2r+1)-(21)=0
We multiply parentheses
6r+3-21=0
We add all the numbers together, and all the variables
6r-18=0
We move all terms containing r to the left, all other terms to the right
6r=18
r=18/6
r=3
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