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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
18/21 = -6/x // + -6/x
18/21-(-6/x) = 0
6*x^-1+18/21 = 0
6*x^-1 = -6/7 // : 6
x^-1 = -1/7
-1 < 0
1/(x^1) = -1/7 // * x^1
1 = -1/7*x^1 // : -1/7
-7 = x^1
x = -7
x = -7
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