Certain is related to District. District is related to Municipal. Municipal is similar to Domestic. 
Tough is related to Strong. Strong is a synonym of Weak. Weaken is derived from Weak. Weaken is a synonym of Fade. 
Certain <--RelatedTo--> District <--RelatedTo--> Municipal <--SimilarTo--> Domestic 
Tough <--RelatedTo--> Strong <--Synonym--> Weak <--DerivedFrom-- Weaken <--Synonym--> Fade 