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x in (-oo:+oo)
4/21 = x/168 // - x/168
4/21-(x/168) = 0
4/21-1/168*x = 0 // - 4/21
-1/168*x = -4/21 // : -1/168
x = -4/21/(-1/168)
x = 32
x = 32
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