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w in (-oo:+oo)
7-(6*w) = 13 // - 13
7-(6*w)-13 = 0
7-6*w-13 = 0
-6*w-6 = 0 // + 6
-6*w = 6 // : -6
w = 6/(-6)
w = -1
w = -1
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