Calculating Equilibrium Concentrations

Consider the reaction A + B ⇌ C + D, for which the equilibrium constant is $K_c = 8.50$ at a certain temperature. If a 1.0 L vessel initially contains 0.300 M of A, 0.400 M of B, 0.100 M of C, and 0.200 M of D, what is the equilibrium concentration of A?