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d in (-oo:+oo)
-d-9 = 15-3*d // - 15-3*d
-d-(-3*d)-15-9 = 0
3*d-d-15-9 = 0
2*d-24 = 0 // + 24
2*d = 24 // : 2
d = 24/2
d = 12
d = 12
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