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a in (-oo:+oo)
-18 = 7*a-(6*a)-6 // - 7*a-(6*a)-6
6*a-(7*a)-18+6 = 0
6*a-7*a-18+6 = 0
-a-12 = 0 // + 12
-a = 12 // * -1
a = -12
a = -12
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