since we have calculated the heat of reaction
assume perfect gas, adiabatic condition, constant volume,
then Cp - Cv = R => Cv = Cp - R
(Cv is heat capacity at constant volume. R is universal gas constant)
dU = heat of reaction
assume zero heat of mixing,
final reaction mixture is 75% propene and 25% cylopropane,
then you arrive at the equation below
Your job is to find the value of T that corresponds to the above equation.
Once you find T, you can use Gay-Lussac's law to find the final Pressure.