Where, D = density of gas = 10.3 g/L P = Pressure of gas =6.43 atm M = Molar mass of gas =39.948 grams/mole R = Gas Law constant = 0.0821 Latm/mol K T = Temperature of gas =?
10.3 = (6.43xx39.948)/(0.0821 xx T)
or, T = 303.76K = 30.61C
Note: 0K = -273.15C so subtract this value from the Kelvin value got above to get Celsius value