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42=7z+21
We move all terms to the left:
42-(7z+21)=0
We get rid of parentheses
-7z-21+42=0
We add all the numbers together, and all the variables
-7z+21=0
We move all terms containing z to the left, all other terms to the right
-7z=-21
z=-21/-7
z=+3
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