Tweak css to not display 2 blocks on mobile devices

Hi all, I'm sure this is a 2 minute job for those in the know - I've got a website that I want to speed up on mobile devices. The simplest way to do this in my head is to hide the large page header block when the screen size is below a certain width.

I've tried, I've given up.

Looking for someone who can add the display: none tag in the right part of the CSS and make it work, because it's doing my head in!