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.