To find the temperature it is necessary to use the expression and concepts related to the ideal gas law.
Mathematically it can be defined as
Where
P = Pressure
V = Volume
n = Number of moles
R = Gas constant
T = Temperature
When the number of moles and volume is constant then the expression can be written as
Or in practical terms for this exercise depending on the final temperature:
Our values are given as
Replacing
Therefore the final temperature of the gas is 800K