Acid Dissociation Constant

A vessel contains 0.5 M acetic acid, which dissociates as shown below. What is the concentration of the acetate ion at equilibrium, given the K##_a## below?
CH##_3##COOH (aq) ##\leftrightharpoons## CH##_3##OO##^{-}## (aq) + H##^+## (aq) , K##_a## = 1.8 x 10##^{-5}##