Assuming that we are given the values of Ka and Kb, these can be rewritten and represented by the concentration of the Hydronium ion and Hydroxide Ion:
Hydronium ion: [H3O+]
Hydroxide ion: [OH-]
Ka is the equilibrium constant of an acid which dissociates to a hydronium ion and its cation.
Kb is the equilibrium constant of a base which dissociates to a hydroxide ion and its anion.
So,
Ka x Kb = [H3O+][OH-]