6 Faces on a Die (singular form of 'Dice')
6 murder weapons in Cluedo
6 faces on a cube
32 Degrees Fahrenheit at which Water Freezes
4 to 6 for dinner
6 Stars in the Flag of Australia
6 Dogs in a Greyhound Race.
28 Days in Four Weeks
32 Degrees Fahrenheit is Freezing Point (of Water)
'6 F of a D' probably means '6 faces of a die' (singular of dice).
18 Minimum Age for a (French/Finnish/Full/Foreign) Driving License
You've given the answer yourself. d=6 and f=3.
72 pints in a firkin
4 Faces on a Pyramid
6 faces on a dice