I assume you meant for the arrow to be as shown below:
Write Keq expression.
Now multiply numerator and denominator by (H^)(OH^-)/(H^+)(OH^-) which is just multiplying by 1 and that doesn't change anything. Now look carefully. Note there is (HNO2)/(H^+)(NO2^-) and that is simply 1/Ka.
Note there is (NH3)/(NH4^+)(OH^-) and that is just 1/Kb.
Look to see what is left and it is (H^+)(OH^-) = Kw.
So the equilibrium constant for the reaction is Kw/KaKb. I will leave it for you to calculate the value. Note that there is NO concn of the salt which means all concns (more or less anyway) give the same pH.