when Keq is a constant which characteristic for every reaction, relates the
concentration of reactants and products at equilibrium at given temperature and
pressure.
and when the Keq expression = concentration of products/concentration of reactants.
Keq = [NO2]^2 / [N2O4]
when [NO2] = 0.0161 M
and [N2O4] = 0.045 M
so by substitution:
Keq= 0.0161^2 / 0.045
= 5.8 x 10^-3