Combustion gases enters an adiabatic gas turbine steadily at 850 kPa and 850oC, with a mass flow

Question:

Combustion gases enters an adiabatic gas turbine steadily at 850 kPa and 850oC, with a mass flow rate of 1 kg/s and leaves at 420 kPa. Treating the combustion gases as air with variable specific heat and assuming an isentropic efficiency of 86 %, determine the work output of the turbine.