I'm having a difficult time understanding f numbers and optics, help explain?

Contact lenses are placed right on the eyeball, so the distance from the eye to an object (or image) is the same as the distance from the lens to that object (or image). A certain person can see distant objects well, but his near point is 44.0cm from his eyes instead of the usual 25.0cm .Is this person nearsighted or farsighted?
I know that the person is farsighted.
What type of lens (converging or diverging) is needed to correct his vision?
They would need a converging lens.
 If the correcting lenses will be contact lenses, what focal length lens is needed ?
1/f= (n-1)[(1/R1)-(1/R2)] where the focal length equals 57.9 cm
What is the power corresponding to the focal length found in part (C) in diopters?
I know that f-number= focal length(f)/ diameter of aperture(D)
I just can't figure out how you would solve for D if you don't know the f number of the eye. Which I really don't think there is one. Maybe I'm missing an equation of a number somewhere?