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81=12u-3u
We move all terms to the left:
81-(12u-3u)=0
We add all the numbers together, and all the variables
-(+9u)+81=0
We get rid of parentheses
-9u+81=0
We move all terms containing u to the left, all other terms to the right
-9u=-81
u=-81/-9
u=+9
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