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x in (-oo:+oo)
(6/7)*x+2/3 = 4*x+212/21 // - 4*x+212/21
(6/7)*x-(4*x)+2/3-(212/21) = 0
(6/7)*x-4*x+2/3-212/21 = 0
-22/7*x-66/7 = 0 // + 66/7
-22/7*x = 66/7 // : -22/7
x = 66/7/(-22/7)
x = -3
x = -3
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