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w in (-oo:+oo)
(5/7)*w+4 = 89/21 // - 89/21
(5/7)*w-(89/21)+4 = 0
(5/7)*w-89/21+4 = 0
5/7*w-5/21 = 0 // + 5/21
5/7*w = 5/21 // : 5/7
w = 5/21/5/7
w = 1/3
w = 1/3
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