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a in (-oo:+oo)
131/168 = (-4/7)*a+(2/3)*a+7/8 // - (-4/7)*a+(2/3)*a+7/8
131/168-((-4/7)*a)-((2/3)*a)-(7/8) = 0
(4/7)*a+(-2/3)*a+131/168-7/8 = 0
-2/21*a-2/21 = 0 // + 2/21
-2/21*a = 2/21 // : -2/21
a = 2/21/(-2/21)
a = -1
a = -1
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