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a in (-oo:+oo)
-(6*a)-6 = 66 // - 66
-(6*a)-66-6 = 0
-6*a-66-6 = 0
-6*a-72 = 0 // + 72
-6*a = 72 // : -6
a = 72/(-6)
a = -12
a = -12
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