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D( x )
13-x = 0
13-x = 0
13-x = 0
13-x = 0 // - 13
-x = -13 // * -1
x = 13
x in (-oo:13) U (13:+oo)
(9*x+6)/(13-x) = 0
9*x+6 = 0 // - 6
9*x = -6 // : 9
x = -6/9
x = -2/3
x = -2/3
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