To solve number 18 you have to add the positive and negative to equal zero. For example for 18a it would be K2S ( the 2 is a subscript ) because you need 2 positives to cancel out the 2 negatives.
18. You can cross over the + - number on the top to each other or make them = 0. a. K2S ........K+ => K 2 from S2.... S2- => S b. CaO........ c. Na2O d. AIN
