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x in (-oo:+oo)
21/84 = x/16 // - x/16
21/84-(x/16) = 0
1/4-1/16*x = 0 // - 1/4
-1/16*x = -1/4 // : -1/16
x = -1/4/(-1/16)
x = 4
x = 4
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