PufferPete

I think I know what I'm doing but I'm confused. Basically I've been given this; K= [NH4+(aq)] X [OH-(aq)] / [NH3(aq)] X [H2O(l)] I've been given the values; K= 3.19x10^-7 NH3(aq)= 1.00x10^-2 mol dm^-3 H2O(l)= 55.4 mol dm^-3 So I need to find the concentration of OH-. I know I have to rearrange the equation BUT since I have already been given a value for K what do I do with the H2O value? I thought I had to move that over to the left. Do I add it to K or multiply it to K?! Or am I way off?! Any handy pointers are greatly appreciated! Cheers Pete.