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a in (-oo:+oo)
-12 = a+2*a+6 // - a+2*a+6
-a-(2*a)-12-6 = 0
-a-2*a-12-6 = 0
-3*a-18 = 0 // + 18
-3*a = 18 // : -3
a = 18/(-3)
a = -6
a = -6
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