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k in (-oo:+oo)
2*(-(5*k)-7) = -84 // + 84
2*(-(5*k)-7)+84 = 0
2*(-5*k-7)+84 = 0
2*(-5*k-7)+84 = 0
70-10*k = 0
70-10*k = 0
70-10*k = 0 // - 70
-10*k = -70 // : -10
k = -70/(-10)
k = 7
k = 7
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