Answer:
There are two possible answers to the question
I) decreasing the pressure of the system
II) decreasing the temperature of the system
Explanation:
Looking at the equation of the reaction, it can be seen that the reaction is endothermic. This implies that decreasing the temperature favours the reverse reaction and more NO2 is produced.
Secondly, decreasing the pressure drives the equilibrium position towards the side having the higher total volume hence decreasing the pressure favours the reverse reaction and more NO2 is produced.