To calculate the number of molecules in a given sample, we need to know what the molecular formula of the substance (which we do), and how many moles of it are present
To calculate the number of moles, we have to divide the mass of the substance (in grams) by its molecular mass
So, in this case, the molecular mass is 55.8 + 3(19) = 112.8
So, the number of moles of FeF3 is = 24/112.8 = 0.213 moles
Once we have the number of moles, we must multiply this value by a number called Avogadro's Constant, which tells us how many atoms/molecules/ions are present in 1 mole of any substance
Avogadro's constant is approximately equal to 6.022 x 10^23
So, there are approximately 0.213 x 6.022 x 10^23 = 1.281 x 10^23 molecules of FeF3 in 24 grams of FeF3
