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x in (-oo:+oo)
21/8 = x/1680 // - x/1680
21/8-(x/1680) = 0
21/8-1/1680*x = 0 // - 21/8
-1/1680*x = -21/8 // : -1/1680
x = -21/8/(-1/1680)
x = 4410
x = 4410
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