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k in (-oo:+oo)
12 = (4/5)*k // - (4/5)*k
12-((4/5)*k) = 0
(-4/5)*k+12 = 0
12-4/5*k = 0 // - 12
-4/5*k = -12 // : -4/5
k = -12/(-4/5)
k = 15
k = 15
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