Explain button function on mobile app

I'm designing an educational Android application, and thus I have icon buttons in my AppBar which have functions like "show answer of the test".

Since there is no default user-used button for this, unlike for "edit this item" function, I think I could either use a custom button Icon, or use the help button Icon.
Anyway, the user won't know at first sight that the function exists except if he has the curiosity to test every buttons, which I think is not the best way.

How can I let the user know about those functions ?

So far I could only think about making a tutorial, but this would take a very long time in my case and is not planned for the moment.