Complex is a synonym of Involved. Involved is related to Tax. Tax is related to Inevitable. Inevitable is a synonym of Fatal. 
Addition is a synonym of Extra. Extra is a synonym of Additional. 
Complex <--Synonym--> Involved <--RelatedTo--> Tax <--RelatedTo--> Inevitable <--Synonym--> Fatal 
Addition <--Synonym--> Extra <--Synonym--> Additional 