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k in (-oo:+oo)
7*(k+8) = -21 // + 21
7*(k+8)+21 = 0
7*(k+8)+21 = 0
7*k+77 = 0
7*k+77 = 0
7*k+77 = 0 // - 77
7*k = -77 // : 7
k = -77/7
k = -11
k = -11
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