“Dry ice” is the solid form of carbon dioxide. Determine the number of mass of CO2 gas in grams that are present in 63.9 L of CO2 at STP.
There is 1 mol CO2 gas in 22.4 L @ STP so in 63.9 L there are 63.9/22.4 = ?
Then you know 1 mol has a mass of 44 g.
Post your work if you get stuck.