Fe is Iron, F is fluorine, for the formula FeF3, there is one molecule of FeF3; this molecule is comprised of one atom of Iron and 3 atoms of fluorine.
Now we need to fine the molecular mass of FeF3, we can do that by looking at the periodic table of element and finding the atomic mass of each element:
Fe = 55.845
F= 18.998
We must multiply the atomic mass of F (18.998) by 3 because there are 3 atoms on F, so we get: 56.994. Now we need to add the atomic mass of Fe (55.845) to the atomic mass of F3 to get: 112.839.
Now you have the weight of FeF3 (112.839). The formula is: Mole = grams/ amu (average atomic mass. The average atomic mass is 112.839 and the question tells us there are 24 grams FeF3, now we divide to get: 0.21 mol/g.
There are 0.21 mol in 24 grams of FeF3, now we need to convert to # of atoms. We can do that by using Avagadro's number: 6.022 • 10^23. We multiply 0.21 by 6.022 • 10^23 to get: 1.26462 • 10^23.
So that would mean that the number of atoms in 24 grams of FeF3 is: 1.26462 • 10^23.
Hope that helped.