Question

what a wonderful city this is! ( change into assertive )

Answer 1

The correct assertive sentence of the given exclamatory sentence is as given below:

This is a very wonderful city.

Explanation

When we convert an exclamatory sentence into an assertive sentence, we remove ‘How’ or ‘What’ and start the sentence with the subject. We put ‘very’ in front of the adjective. For example, the exclamatory sentence ‘How intelligent she is!’ will change into ‘She is very intelligent.’

Answer 2

Well, the answer of your question is " This is a wonderful city"
actually the assertive sentence are the one in which a fact is stated normally and are quite simple sentence . So in order to change the sentence into assertive the "what'' , how etc are removed and sentence start with subject . This is why we start this sentence with subject and remove the What from the question then align the sentence in  order to make it simpler.