I don't know if you already know about the mole hold but basically you must convert into moles to get to another thing there are no shortcuts.
First you must divide 4.8x10^24 by avagadros number(6.02x10^23) doing this will convert 4.8x10^24 atoms into moles
Next take the moles and multiply it by the molar mass of strontium to get the mass in grams
(4.8x10^24)/(6.02x10^23)=7.973421927moles
Multiply the number of mole by the molar mass of strontium(88g/mol)
7.973421927x88=701.67g