Calculating value of equilibrium constant

Question

Assignment:

Calculate the value of the equilibrium constant for each of the following reactions ( we abbreviate citric acid as H₃Cit) using the Ka values for citric acid and for carbonic acid:

H₃Cit + HCO₃- <--> H₂Cit- + H₂O +CO₂
H₃Cit + 3HCO₃- <--> Cit₃- + 3H₂O +3CO₂
Given Ka₁ of H₃Cit =8.4x10^-4; Ka₂ of H₃Cit = 1.8x10^-5; Ka₃ of H₃Cit = 4.0x10^-6; Ka₁ of H₂CO₃ = 4.3x10^-7; Ka₂ of H₂CO₃ = 5.6x10^-11

Hint: When reactions are added, the corresponding equilibrium constants are multiplied.

The K_a values are known for the H₃Cit and its equilibrium. How/where is the HCO₃^- Ka value found/calculated to include it into the equation. Also, the same question with the H₂O and the CO₂. These are not acids so how is this a part of this set up.

Provide complete and step by step solution for the question and show calculations and use formulas.