For questions 11-15
11. Magnesium Oxide is a combination between Magnesium and oxygen so the formula will be MgO
12. Calcium Chloride is a combination between Calcium and Chlorine, so the formula will be CaCl₂ the ₂ comes from the fact that there is two Chlorine atoms
13. Lithium Sulfide is a combination between Two lithium and a sulfur atoms,
so the formula will be Li₂S The ₂ coming from the two lithium atoms
14. Barium Selenide is a combination between Barium and Selenium so the formula will be BaSe
15. Aluminum Oxide is a combination between Aluminum and Oxygen, there are two Aluminum atoms and three Oxygen atoms, so it will make Al₂O₃
Hope this helps