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f in (-oo:+oo)
(1/4)*(f+3) = 6 // - 6
(1/4)*(f+3)-6 = 0
1/4*(f+3)-6 = 0
1/4*(f+3)-6 = 0
1/4*f-21/4 = 0
1/4*f-21/4 = 0
1/4*f-21/4 = 0 // + 21/4
1/4*f = 21/4 // : 1/4
f = 21/4/1/4
f = 21
f = 21
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