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x in (-oo:+oo)
x-2*x+3/7 = -81/7 // + -81/7
x-2*x+3/7-(-81/7) = 0
x-2*x+3/7+81/7 = 0
12-x = 0 // - 12
-x = -12 // * -1
x = 12
x = 12
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