In order to solve this problem, you will need to be aware of a number called "Avogadro's number." Avogadro's number, 6.022⋅1023, details the number of molecules/atoms in 1 mole of substance. Keeping this in mind, here is the dimensional analysis setup for this particular question (we are converting from moles S2O3 to molecules of S2O3 (using Avogadro's number) to number of atoms).
0.67 mol S2O3⋅(6.022⋅1023 molecules S2O31 mole of S2O3)⋅(5 atoms1 molecule S2O3)=2.0⋅1024 atoms
Remember that I could have messed up the actual calculations, so I recommend that you type it into your calculator once again to check.
In case you were confused where I got the 5 from, S2O3 has 2 S atoms and 3 O atoms. Since 2+3=5, this tells us that one molecule of S2O3 has 5 atoms.
I hope that helps!
