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k in (-oo:+oo)
7*(-3*k-7) = -133 // + 133
7*(-3*k-7)+133 = 0
7*(-3*k-7)+133 = 0
84-21*k = 0
84-21*k = 0
84-21*k = 0 // - 84
-21*k = -84 // : -21
k = -84/(-21)
k = 4
k = 4
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