both parts a and b please, thank you!

3. (a) Consider hexagonal close-packing (HCP) of oxygen ions. What is the ratio of number of ions in octahedral sites to oxygen ions in a unit cell? What is the ratio of number of ions in tetrahedral sites to oxygen ions in a unit cell? (b) Explain what valence of cations are required to have stable structures in which: 1. All octahedral sites are filled. 2. Two third the octahedral sites are filled. 3. Half the tetrahedral sites are filled. Give an example of each