The problem probably lies in the fact that <textarea> and the submit button are both form controls and inline elements. There are two ways to do what you want; maybe more.
One is to convert <textarea> and <input> to block level elements like this:
I have the two elements wrapped in the <p> tag with the width set there. The text area becomes a block element the same width as the <p> with the submit button centered in the <p> also.
The second way could be done using inline-block but I leave that as an exercise because I'm too lazy to do it.
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