Could is the opposite of Can. Drum is a type of Can. Drum is related to Musical. 
Beautiful is the opposite of Average. Average is a synonym of Conventional. Conventional is related to Formal. 
Could <--Antonym--> Can <--IsA-- Drum <--RelatedTo--> Musical 
Beautiful <--Antonym--> Average <--Synonym--> Conventional <--RelatedTo--> Formal 