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D in (-oo:+oo)
9/0.2-(3*D) = 2.1 // - 2.1
9/0.2-(3*D)-2.1 = 0
9/0.2-3*D-2.1 = 0
42.9-3*D = 0 // - 42.9
-3*D = -42.9 // : -3
D = -42.9/(-3)
D = 14.3
D = 14.3
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