Difficult is related to Tough. Tough is the opposite of Gentle. Gentle is the opposite of Harsh. Harsh is related to Cruel. 
Bill is related to Check. Check is related to Payment. Benefit is a type of Payment. Benefit is a synonym of Use. 
Difficult <--RelatedTo--> Tough <--Antonym--> Gentle <--Antonym--> Harsh <--RelatedTo--> Cruel 
Bill <--RelatedTo--> Check <--RelatedTo--> Payment <--IsA-- Benefit <--Synonym--> Use 