The word 'tranquil' is an adjective. It describes a state of calmness, of freedom from agitation, disturbance, or stress:'The beach was tranquil, with its calm sea and slight breeze.''The person was...
No, it takes an adverb to do that. In the phrase a red car, the word red is clearly an adjective. In a big red car, we have two adjectives, both describing the noun, the car. So a bright red car also...