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d in (-oo:+oo)
2*(d+6) = -6 // + 6
2*(d+6)+6 = 0
2*(d+6)+6 = 0
2*d+18 = 0
2*d+18 = 0
2*d+18 = 0 // - 18
2*d = -18 // : 2
d = -18/2
d = -9
d = -9
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