Need help with hiding menu and opt in box

Hello, I’m helping a friend with her website and I’m trying to hide the horizontal opt in box and menu from this page:
https://annathea.org/freeclass

and I’ve figured out that the horizontal box is in the css and I really don’t know anything about css. Can anyone help? I need to do the same on this page too.
https://annathea.org/emotions

I would be grateful if someone can help me figure out how to do that and where to put the code. Thanks in advance for your help!