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d in (-oo:+oo)
-4*d-3 = 5*d-21 // - 5*d-21
21-4*d-(5*d)-3 = 0
21-4*d-5*d-3 = 0
18-9*d = 0 // - 18
-9*d = -18 // : -9
d = -18/(-9)
d = 2
d = 2
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