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D( x )
-3*x-7 = 0
-3*x-7 = 0
-3*x-7 = 0
-3*x-7 = 0 // + 7
-3*x = 7 // : -3
x = 7/(-3)
x = -7/3
x in (-oo:-7/3) U (-7/3:+oo)
-5/(-3*x-7) = 0
x belongs to the empty set
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