Answer:
81.3 g/mol 
Step-by-step explanation:
STP is 0 °C and 1 bar. 
Under these conditions, 1 mol of a gas occupies 22.71 L. 
Mass = volume × density = 22.71 L × 3.58 g/L = 81.3 g 
The molar mass of the gas is 81.3 g/mol. 
Are you using the old (before 1982) definition of STP? Under that definition, the molar mass of the gas would be 80.2 g/mol.