What is the exit pressure that will allow a reversible subsonic exit flow in the previous problem?

Data from previous problem

Air flows into a convergent-divergent nozzle with an exit area 2.0 times the throat area of 0.005 m². The inlet stagnation state is 1.2 MPa, 600 K. Find the back pressure that will cause a reversible supersonic exit flow with M = 1 at the throat. What is the mass flow rate?