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x in (-oo:+oo)
142/21 = (-1/6)*x+(2/7)*x+7 // - (-1/6)*x+(2/7)*x+7
142/21-((-1/6)*x)-((2/7)*x)-7 = 0
(1/6)*x+(-2/7)*x+142/21-7 = 0
-5/42*x-5/21 = 0 // + 5/21
-5/42*x = 5/21 // : -5/42
x = 5/21/(-5/42)
x = -2
x = -2
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