when co2 dissolves in water, the following equillibrium is established, CO2 + 2H2O GIVES H3O(+) + HCO3(-) for which the equillibrium constant is 3.8 * 10^-7 and pH is 6. then what is the ratio of concentration HCO3(-) to CO2?

CO2 2H2O ==> H3O^+ + HCO3^-

K = (H3O^+)(HCO3^-)/(CO2)
[K/(H3O^+)] = [(HCO3^-)/(CO2)]
Substitute for K and (H3O^+) and you have the answer.