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k in (-oo:+oo)
5*k+8 = 104-7*k // - 104-7*k
5*k-(-7*k)-104+8 = 0
5*k+7*k-104+8 = 0
12*k-96 = 0 // + 96
12*k = 96 // : 12
k = 96/12
k = 8
k = 8
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