Hi!
To convert grams to moles, you need to divide the amount (grams) of your substance by its molar mass, both of which are given to you.
28 g CaO / 58.08 g/mol CaO = 0.4820936639 moles of CaO
(And 28 contains two significant digits, so you will round this to 0.48 moles.)
Good luck!
