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k in (-oo:+oo)
7-(7*k) = 21 // - 21
7-(7*k)-21 = 0
7-7*k-21 = 0
-7*k-14 = 0 // + 14
-7*k = 14 // : -7
k = 14/(-7)
k = -2
k = -2
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