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k in (-oo:+oo)
(2/5)*k+(3/7)*k+5 = 88/35 // - 88/35
(2/5)*k+(3/7)*k-(88/35)+5 = 0
(2/5)*k+(3/7)*k-88/35+5 = 0
29/35*k+87/35 = 0 // - 87/35
29/35*k = -87/35 // : 29/35
k = -87/35/29/35
k = -3
k = -3
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