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-x-21+4x+21=-4x+4x+21
We move all terms to the left:
-x-21+4x+21-(-4x+4x+21)=0
We add all the numbers together, and all the variables
-x+4x-21+21-21=0
We add all the numbers together, and all the variables
3x-21=0
We move all terms containing x to the left, all other terms to the right
3x=21
x=21/3
x=7
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