Question

change the form of narration the girl said I shall do it​

Answer 1

$\bf{Answer:}$

The girl said/told that she would do it.

Answer 2

The answer is: The girl asserted that she (the girl) will do that. Here the narration is done in a way that the girl is asserting something to do.

Here the meaning if the sentence is kept same whereas it has been described in the way similar to the girl’s speech.  

The subject verb agreement and use of tense is kept the same.