Question

What is the equilibrium constant of the reaction 2NO2 (g) with 3.0 M and N2O4 with 2.5M

Answer 1

If those are equilibrium concentrations and the reaction is as below, then
2NO2 ==> N2O4

Keq = (N2O4)/(NO2)^2 = ??
Plug in the equilibrium concentrations and calculate.