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21/r=7
We move all terms to the left:
21/r-(7)=0
Domain of the equation: r!=0We multiply all the terms by the denominator
r∈R
-7*r+21=0
We add all the numbers together, and all the variables
-7r+21=0
We move all terms containing r to the left, all other terms to the right
-7r=-21
r=-21/-7
r=+3
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