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k in (-oo:+oo)
(1/4)*k+(6/7)*k-3/4 = -393/28 // + -393/28
(1/4)*k+(6/7)*k-3/4-(-393/28) = 0
(1/4)*k+(6/7)*k-3/4+393/28 = 0
31/28*k+93/7 = 0 // - 93/7
31/28*k = -93/7 // : 31/28
k = -93/7/31/28
k = -12
k = -12
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