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k in (-oo:+oo)
5*(6-(3*k)) = -120 // + 120
5*(6-(3*k))+120 = 0
5*(6-3*k)+120 = 0
5*(6-3*k)+120 = 0
150-15*k = 0
150-15*k = 0
150-15*k = 0 // - 150
-15*k = -150 // : -15
k = -150/(-15)
k = 10
k = 10
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