Percent Ionization

The percent ionization of NH##_3## is found to be 4.2% in a solution, find the concentration of NH##_3##.
NH##_3## (aq) + H##_2##O ##\leftrightharpoons## NH##_4^+## + OH##^-## , K##_b## = 1.8 x 10##^{-5}##