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k in (-oo:+oo)
k+3 = 7*k+15 // - 7*k+15
k-(7*k)-15+3 = 0
k-7*k-15+3 = 0
-6*k-12 = 0 // + 12
-6*k = 12 // : -6
k = 12/(-6)
k = -2
k = -2
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