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a in (-oo:+oo)
(2/7)*a-((1/6)*a)+3 = 73/21 // - 73/21
(2/7)*a-((1/6)*a)-(73/21)+3 = 0
(2/7)*a+(-1/6)*a-73/21+3 = 0
5/42*a-10/21 = 0 // + 10/21
5/42*a = 10/21 // : 5/42
a = 10/21/5/42
a = 4
a = 4
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