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k in (-oo:+oo)
(23-k)/21 = 18 // - 18
(23-k)/21-18 = 0
(23-k)/21+(-18*21)/21 = 0
23-k-18*21 = 0
-k-355 = 0
(-k-355)/21 = 0
(-k-355)/21 = 0 // * 21
-k-355 = 0
-k-355 = 0 // + 355
-k = 355 // * -1
k = -355
k = -355
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