Well, Let's begin by writing out the system of reaction equations:
CO2 + H2O -> H2CO3
H2CO3 <-> HCO3- + H+
HCO3- + H+ + NaOH -> HCO3- + H2O + Na+
HCO3- + NaOH -> CO32- + H2O + Na+
write the equilibrium equations out for these reactions, make sure you use the proper Ka values, then use this system of equations to solve for H2CO3. That there should put you on the right track. Try it out, if you get hung up, post what you've done and your ?'s and we'll work from there.