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x in (-oo:+oo)
6-(2*x) = x+15 // - x+15
6-(2*x)-x-15 = 0
6-2*x-x-15 = 0
-3*x-9 = 0 // + 9
-3*x = 9 // : -3
x = 9/(-3)
x = -3
x = -3
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