Answer
${CO_3}^{2-} + H_2O -> {HCO_3}^- + OH^-$
$Kb = \frac{[{HCO_3}^-][OH^-]}{[{CO_3}^{2-}]}$
Work Step by Step
1. Write the reaction with water:
${CO_3}^{2-} + H_2O -> {HCO_3}^- + OH^-$
2. Now, write the Kb formula:
$Kb = \frac{[Conjugate.acid]*[OH^-]}{[Base]}$
$Kb = \frac{[{HCO_3}^-][OH^-]}{[{CO_3}^{2-}]}$