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x in (-oo:+oo)
(-1/6)*(x-12)-((1/2)*(x-2)) = x-21 // - x-21
(-1/6)*(x-12)-((1/2)*(x-2))-x+21 = 0
(-1/6)*(x-12)+(-1/2)*(x-2)-x+21 = 0
21-1/6*(x-12)-1/2*(x-2)-x = 0
21-1/6*(x-12)-1/2*(x-2)-x = 0
3-2/3*x-x+21 = 0
3-5/3*x+21 = 0
24-5/3*x = 0
24-5/3*x = 0
24-5/3*x = 0 // - 24
-5/3*x = -24 // : -5/3
x = -24/(-5/3)
x = 72/5
x = 72/5
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