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21=2v+5v
We move all terms to the left:
21-(2v+5v)=0
We add all the numbers together, and all the variables
-(+7v)+21=0
We get rid of parentheses
-7v+21=0
We move all terms containing v to the left, all other terms to the right
-7v=-21
v=-21/-7
v=+3
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