2MeNO3=2MeNO2+O2

2                                    1

m(O2)=6,06-5,1=0,96г;

n(O2)=0,96:32=0,03моль; n(MeNO3)=0,03*2=0,06моль; M(MeNO3)=6,06:0.06=101г/моль;

                                                         МеNO3

                                                     x+14+48=101

                                                              x=39;

ответ:калий.