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x in (-oo:+oo)
13 = x+(2/3)*x+8 // - x+(2/3)*x+8
13-x-((2/3)*x)-8 = 0
(-2/3)*x-x-8+13 = 0
5-5/3*x = 0 // - 5
-5/3*x = -5 // : -5/3
x = -5/(-5/3)
x = 3
x = 3
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