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a in (-oo:+oo)
5/6-((2/7)*a) = (-6/7)*a+23/42 // - (-6/7)*a+23/42
5/6-((2/7)*a)-((-6/7)*a)-(23/42) = 0
(-2/7)*a+(6/7)*a+5/6-23/42 = 0
4/7*a+2/7 = 0 // - 2/7
4/7*a = -2/7 // : 4/7
a = -2/7/4/7
a = -1/2
a = -1/2
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