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a in (-oo:+oo)
(5/7)*a+(3/5)*a+3/7 = 73/63 // - 73/63
(5/7)*a+(3/5)*a+3/7-(73/63) = 0
(5/7)*a+(3/5)*a+3/7-73/63 = 0
46/35*a-46/63 = 0 // + 46/63
46/35*a = 46/63 // : 46/35
a = 46/63/46/35
a = 5/9
a = 5/9
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