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-3(r+4)=21
We move all terms to the left:
-3(r+4)-(21)=0
We multiply parentheses
-3r-12-21=0
We add all the numbers together, and all the variables
-3r-33=0
We move all terms containing r to the left, all other terms to the right
-3r=33
r=33/-3
r=-11
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