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k in (-oo:+oo)
-(3*k)-3 = -39 // + 39
39-(3*k)-3 = 0
39-3*k-3 = 0
36-3*k = 0 // - 36
-3*k = -36 // : -3
k = -36/(-3)
k = 12
k = 12
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