This is a quick tip!
Là-dessus means on this/it, and can be used in the literal sense, meaning on a surface.
Vous pouvez déposer votre manteau là-dessus
You can put your your coat on this
It can also mean, figuratively, on it or about a certain topic.
Qu’est-ce qu’on n’a pas écrit là-dessus?
What haven’t we written about this?