Where does king seek to establish that he is a reasonable person?

At the beggining of "Letter From Birmingham Jail".

Explanation:

King establishes this in the first paragraph.

" Seldom, if ever, do I pause to answer criticism of my work and ideas. If I sought to answer all of the criticisms that cross my desk, my secretaries would be engaged in little else in the course of the day, and I would have no time for constructive work. But since I feel that you are men of genuine good will and your criticisms are sincerely set forth, I would like to answer your statement in what I hope will be patient and reasonable terms."