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a in (-oo:+oo)
(1/6)*a-(3*a)-3/4 = 5/84 // - 5/84
(1/6)*a-(3*a)-3/4-(5/84) = 0
(1/6)*a-3*a-3/4-5/84 = 0
-17/6*a-17/21 = 0 // + 17/21
-17/6*a = 17/21 // : -17/6
a = 17/21/(-17/6)
a = -2/7
a = -2/7
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