You have to use Avogadro's number which is 6.02×10²³ which tells you the number of particles in 1 mole of a sample.
6a) 2mol Fe²⁺
(2mol Fe²⁺)×(6.02×10²³ions/mol)=1.2×10²⁴ ions of Fe²⁺
6b) 4.5mol BCl₃
(4.5mol BCl₃)×(6.02×10²³ molecules/mol)=2.7×10²⁴molecules of BCl₃
I hope this helps. Let me know if anything is unclear.