Answer
(a) CaS
(b) Mg$_3$N$_2$
(c) FeF$_2$
Work Step by Step
(a) CaS: Calcium with a valence of 2+ and sulfur with a valence of 2- combine in a 1:1 ratio, meaning we will need one calcium atom for every sulfur atom to the balance the charges in this compound.
(b) Mg$_3$N$_2$: Magnesium has a valence of 2+, and nitrogen has a valence of 3-. We need three magnesium atoms for every two nitrogen atoms to balance the charges in this compound.
(c) FeF$_2$: We see that , in this problem, iron has a valence of 2+. Fluorine is a halogen and has a valence of 1-. We will need one iron atom for every two fluorine atoms to balance the charges in this compound.
