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h in (-oo:+oo)
5/21 = h/84 // - h/84
5/21-(h/84) = 0
5/21-1/84*h = 0 // - 5/21
-1/84*h = -5/21 // : -1/84
h = -5/21/(-1/84)
h = 20
h = 20
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