Just use dimensional analysis to see how it works. There are 6.02 x 10^23 molecules in a mole of molecules and there are 3 F atoms in the C2HBrClF3 molecule. So start with what you have and use factors to get where you want to go.
12.15 moles Cx x factor x factor = # F atoms.
12.15 moles Cx x (6.02 x 10^23 molecules Cx/1 mol Cx) x (3 F atoms/1 molecule Cx) = ??
Note that moles Cx (in the 12.15) cancel with the mol Cx in the first factor and molecules Cx in the numerator of the first factor cancel with molecules Cx in the denominator of factor 2, leaving F atoms which is the unit you want to keep.