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42=3u+15
We move all terms to the left:
42-(3u+15)=0
We get rid of parentheses
-3u-15+42=0
We add all the numbers together, and all the variables
-3u+27=0
We move all terms containing u to the left, all other terms to the right
-3u=-27
u=-27/-3
u=+9
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