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3*x+9 = 21/2 // - 21/2
3*x-(21/2)+9 = 0
3*x-21/2+9 = 0
3*x-3/2 = 0 // + 3/2
3*x = 3/2 // : 3
x = 3/2/3
x = 1/2
x = 1/2
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