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a in (-oo:+oo)
6*a-(1/7) = 221/63-((4/7)*a) // - 221/63-((4/7)*a)
6*a+(4/7)*a-(1/7)-(221/63) = 0
6*a+(4/7)*a-1/7-221/63 = 0
46/7*a-230/63 = 0 // + 230/63
46/7*a = 230/63 // : 46/7
a = 230/63/46/7
a = 5/9
a = 5/9
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