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x in (-oo:+oo)
(2/5)*x-3 = f // - f
(2/5)*x-f-3 = 0
2/5*x-f-3 = 0 // + -f-3
2/5*x = -(-f-3) // : 2/5
x = (-(-f-3))/2/5
x = 5/2*(f+3)
x = 5/2*(f+3)
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