With the information given, you first determine the moles of benzene that can be melted with 49.8kJ or energy by dividing that number by the heat of fusion for benzene, 9.95kJ/mol. This division will tell you how many moles of benzene can be melted.
(49.8kJ) / (9.95kJ/mol) = 5.01 mol
You convert these moles to grams by multiplying by the molar mass of benzene, which is around 78.11g/mol. So...
5.01mol * 78.11g/mol = 390.95 g
When you round for sig figs, you get 391g of benzene can melted.