Equilibrium Ice Tables

0.800 moles of N##_2## (g) are placed in a 7.00 L flask with 3.00 moles of H##_2## and allowed to react for some time. After several hours the concentrations of H##_2## (g) is measured at 0.278 M. What is the equilibrium constant for the reaction at this temperature?
N##_2## (g) + 3 H##_2## (g) ##\leftrightharpoons## 2 NH##_3## (g)