In order to calculate the final temperature of the gas, we may apply Charles's law, which states that the pressure and temperature of a fixed amount of gas at constant volume are directly proportional. Mathematically:
P/T = constant
(absolute temperature is used, so T = 672 + 273 = 945 K)
Thus,
3.9 / 945 = 12.2 / T
T = 2,956 K or 2,683 °C