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x in (-oo:+oo)
-18 = (2/3)*(6*x-21) // - (2/3)*(6*x-21)
-((2/3)*(6*x-21))-18 = 0
(-2/3)*(6*x-21)-18 = 0
-2/3*(6*x-21)-18 = 0
-2/3*(6*x-21)-18 = 0
-4*x-4 = 0
-4*x-4 = 0
-4*x-4 = 0 // + 4
-4*x = 4 // : -4
x = 4/(-4)
x = -1
x = -1
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