If your back and next buttons are on the Master, you'll want to use Global Events to do this. If you're using one of our Navigation templates, the functionality is already there - see this forum post for more info: https://portal.smartbuilder.com/web/smartbuilder/forum/-/message_boards/message/394589#_19_message_394538
Each Global Event has 2 parts - you fire the Global Event when need to (say when the page loads, or when they have finished the activity on the page), and then you "catch" it (usually on the Master Page, and do the thing you want to do).
If you're not using one of our templates, then you'll have to create those actions that "catch" the Global Events on your Master shown in the other forum post.
When to trigger the "enable next" Global Event will probably be a conditional action that has multiple triggers - when each of the fact buttons are clicked (see attached image).
That is as specific as we can be without seeing the actual lesson. Hope that helps!