since chlorine is one of the 7 diatomic elements we know that chlorine appears as Cl₂ gas naturally. That means that the molar mass of a chlorine gas is 70.9g/mol. That being said, first you need to find the number of moles of chlorine gas that are present in a 35.5g sample. To do this divide 35.5g by the molar mass of chlorine gas (70.9g/mol) to get 0.501mol of chlorine. Then you have to multiply 0.501mol by 6.02×10²³ to get the number of chlorine gas molecules. Therefore 3.01×10²³ molecules of chlorine gas are present in a 35.5g sample.
I hope that helps. Let me know in the comments if anything is unclear.