Well, Let's begin by writing out the system of reaction equations:

CO_{2} + H_{2}O -> H_{2}CO_{3}

H_{2}CO_{3} <-> HCO_{3}^{-} + H^{+}

HCO_{3}^{-} + H^{+} + NaOH -> HCO_{3}^{-} + H_{2}O + Na^{+}

HCO_{3}^{-} + NaOH -> CO_{3}^{2-} + H_{2}O + 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 H_{2}CO_{3}. 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.