Respuesta :

NO2- +7H+ + 6e->> NH3 + 2H2O ( N goes from +3 to -3) 

Al + 2 H2O >> AlO2- + 4H+ + 3e- 

NO2- + 2Al + 2H2O >> NH3 + 2AlO2- + H+ 

add OH- on the left and on the right 

NO2- + 2 Al + H2O + OH- >> NH3 + 2 AlO2-