I’m developing a web application that in a view has three panels as the image shows:

Which CSS properties would you use to have the bottom panel get the same width as the upper two panels combined in a dynamic context?
EDIT: I’m sorry I should have mentioned that I can’t use any javascript. I need a solution purely executed by CSS. The markup is generated from the java code of which I’m developing the application so I don’t have a direct access to that either.
Learn about
display:inline-block;,float,positionand other properties for display and positioning options. I just guess there’s something like a wrapper.HTML:
CSS:
Demo