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k in (-oo:+oo)
7-6*k-(5*k) = 40 // - 40
7-6*k-(5*k)-40 = 0
7-6*k-5*k-40 = 0
-11*k-33 = 0 // + 33
-11*k = 33 // : -11
k = 33/(-11)
k = -3
k = -3
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