I have no idea what this puzzle means or says

I have three of the four letters, am missing the one I’m assuming is associated with code 53-can you guide me In the right direction?

I have three of the four letters, am missing the one I’m assuming is associated with code 53-can you guide me In the right direction?

Can someone please give me a hint? I’ve plotted the elements ive found in the book.

I need a hint for puzzle 20 please-can’t figure out how the letters line up