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x in (-oo:+oo)
21-(x/6) = x/12 // - x/12
21-(x/6)-(x/12) = 0
21-1/4*x = 0 // - 21
-1/4*x = -21 // : -1/4
x = -21/(-1/4)
x = 84
x = 84
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