First you need to find out how many moles of AsF3 are in 1.92 grams.
The molar mass of AsF3 is 131.918 g/mole so
1.92 g * (1 mole)/(131.918g) = .0146 moles AsF3
Each mole of AsF3 has 6.02 x10^23 molecules of AsF3 and each molecule of AsF3 has 3 flourines, so you need to convert from moles to molecules AsF3 to atoms of F
0.146 moles AsF3 * (6.02 x 10^23 molecules AsF3)/(1 mole AsF3) * (3 atoms F)/(1 molecule AsF3) = 2.64 x 10^22 atoms F
