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42=3u+12
We move all terms to the left:
42-(3u+12)=0
We get rid of parentheses
-3u-12+42=0
We add all the numbers together, and all the variables
-3u+30=0
We move all terms containing u to the left, all other terms to the right
-3u=-30
u=-30/-3
u=+10
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