Reducing left margin on post page

Hi,
On twenty eleven – the text in my static post page has a wide left margin, or maybe it’s an issues with the divs. In any case, I’d like the the text in the post to be aligned left with the date at the top.
https://social-way.com/really/

Which part of the CSS controls this?
How do you generally figure out which part of the CSS controls the different sections of the site?

I’m using a child theme BTW.

Thanks!