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a in (-oo:+oo)
(a-6)/12 = (a-2)/3 // - (a-2)/3
(a-6)/12-((a-2)/3) = 0
(a-6)/12+(-1*(a-2))/3 = 0
(3*(a-6))/(3*12)+(-1*12*(a-2))/(3*12) = 0
3*(a-6)-1*12*(a-2) = 0
6-9*a = 0
(6-9*a)/(3*12) = 0
(6-9*a)/(3*12) = 0 // * 3*12
6-9*a = 0
6-9*a = 0 // - 6
-9*a = -6 // : -9
a = -6/(-9)
a = 2/3
a = 2/3
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