Answer
See the answer below.
Work Step by Step
a) A sample of an ideal gas was initially under a pressure of 500 kPa when its temperature is 400$^\circ$C. It then undergoes an isochoric process where its final pressure is 200 kPa. Find its final temperature in Celsius.
b) We have 2 points here, $(V,500\;\rm kPa)$, and $(V,200\;\rm kPa)$
See the figure below.
c) The final temperature is given by
$$T_2=\dfrac{200(400+273)}{500}-273$$
$$T_2=\color{red}{\bf -3.8}^\circ \rm C$$
