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k in (-oo:+oo)
3/5-3*k = 57/20 // - 57/20
3/5-3*k-(57/20) = 0
3/5-3*k-57/20 = 0
-3*k-9/4 = 0 // + 9/4
-3*k = 9/4 // : -3
k = 9/4/(-3)
k = -3/4
k = -3/4
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