The equilibrium constant at 850 K for the reaction $\mathrm{N_2(g) + O_2(g) \rightleftharpoons 2NO(g)}$ is 0.5625. The equilibrium concentration of NO(g) is $3.0 \times 10^{-3}$ M. If the equilibrium concentrations of N$_2$(g) and O$_2$(g) are equal, the concentration of N$_2$(g) in M is: