Tough is the opposite of Weak. Weak is the opposite of Powerful. Powerful is a synonym of Strong. 
Deliver is related to Brown. Brown is related to Dirt. Dirt is related to Pollution. Pollution is related to Emission. 
Tough <--Antonym--> Weak <--Antonym--> Powerful <--Synonym--> Strong 
Deliver <--RelatedTo--> Brown <--RelatedTo--> Dirt <--RelatedTo--> Pollution <--RelatedTo--> Emission 