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d in (-oo:+oo)
6-(7*d) = 13 // - 13
6-(7*d)-13 = 0
6-7*d-13 = 0
-7*d-7 = 0 // + 7
-7*d = 7 // : -7
d = 7/(-7)
d = -1
d = -1
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