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f in (-oo:+oo)
21 = (5/9)*(f-32) // - (5/9)*(f-32)
21-((5/9)*(f-32)) = 0
(-5/9)*(f-32)+21 = 0
21-5/9*(f-32) = 0
21-5/9*(f-32) = 0
349/9-5/9*f = 0
349/9-5/9*f = 0
349/9-5/9*f = 0 // - 349/9
-5/9*f = -349/9 // : -5/9
f = -349/9/(-5/9)
f = 349/5
f = 349/5
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