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f in (-oo:+oo)
(7/5)*(f+6) = 2/5-f // - 2/5-f
(7/5)*(f+6)+f-(2/5) = 0
(7/5)*(f+6)+f-2/5 = 0
7/5*(f+6)+f-2/5 = 0
(7/5*5*(f+6))/5+(5*f)/5-2/5 = 0
7/5*5*(f+6)+5*f-2 = 0
12*f-2+42 = 0
12*f+40 = 0
(12*f+40)/5 = 0
(12*f+40)/5 = 0 // * 5
12*f+40 = 0
12*f+40 = 0 // - 40
12*f = -40 // : 12
f = -40/12
f = -10/3
f = -10/3
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