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45=4r+21
We move all terms to the left:
45-(4r+21)=0
We get rid of parentheses
-4r-21+45=0
We add all the numbers together, and all the variables
-4r+24=0
We move all terms containing r to the left, all other terms to the right
-4r=-24
r=-24/-4
r=+6
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