Question

Calculate the molar concentration of uncomplexed Zn2+ (aq) in a solution that contains 0.22 mol of Zn(NH3)4 2+ per liter and 0.3109 M NH3 at equilibrium. Kf for Zn(NH3)4 2+ is
2.9 X 10^9

I started this way...

(0.3109 + 4)2/0.22-x)= 2.9 X 10^9

I'm lost and cant figure out how to do this problem