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a in (-oo:+oo)
3*a-(6*a)+4 = 40 // - 40
3*a-(6*a)-40+4 = 0
3*a-6*a-40+4 = 0
-3*a-36 = 0 // + 36
-3*a = 36 // : -3
a = 36/(-3)
a = -12
a = -12
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