What is the assertive form of the sentence "What an excellent idea"?

• What an excellent idea! - এটি একটি Exclamatory Sentence. 

• Exclamatory Sentence কে Assertive Sentence - এ রূপান্তর করার নিয়ম: 
Subject + verb + a/an + very/great + adjective + বাকী অংশ। 

Exclamatory Sentence: What an excellent idea!
Assertive Sentence: It is a very excellent idea.

More example: 
Exclamatory Sentence: What a remarkable achievement! 
Assertive Sentence: It is a very remarkable achievement.

Source: Advanced Learner's English Grammar & Composition by Chowdhury & Hossain.