I am not sure how to answer the question below

Question

Calculate the molar concentration of uncomplexed Zn2+(aq) in a solution that contains 0.17 M Zn(NH3)42+ and 0.3775 M NH3 at equilibrium. Kf for Zn(NH3)42+ is .

Answer 1

Zn²⁺(aq) + 4NH₃(aq) <==> Zn(NH₃)₄²⁺(aq) ... Kf = 2.9x10⁹

Kf = [Zn(NH₃)₄²⁺] / [Zn²⁺][NH₃]⁴

2.9x10⁹ = (0.17) / (Zn²⁺)(0.3775)⁴

[Zn²⁺] = 0.17 / (2.9x10⁹)(0.3775)⁴

[Zn²⁺] = 2.9x10^-9 M

(Be sure to check my math)