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D( x )
3-x = 0
3-x = 0
3-x = 0
3-x = 0 // - 3
-x = -3 // * -1
x = 3
x in (-oo:3) U (3:+oo)
6 = x/(3-x) // - x/(3-x)
6-(x/(3-x)) = 0
(-1*x)/(3-x)+6 = 0
(-1*x)/(3-x)+(6*(3-x))/(3-x) = 0
6*(3-x)-1*x = 0
18-7*x = 0
(18-7*x)/(3-x) = 0
(18-7*x)/(3-x) = 0 // * 3-x
18-7*x = 0
18-7*x = 0 // - 18
-7*x = -18 // : -7
x = -18/(-7)
x = 18/7
x = 18/7
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