Watch this 5-minute video, copy the example onto notes, then glue the notes into your notebook. Think about what data you will need to collect to find the empirical formula of the magnesium oxide you create in the lab.
For more help, study the example on p. 252 of the textbook (p.264 in ebook)
If you have not mastered empirical formulas, practice a few problems on this interactive website: http://tinyurl.com/z6uc9vd
If you still need help, schedule a workshop.