Now, going backwards, the enzyme and the substrate reassociate to form back the complex. So this is the rate constant going this way. This is the rate constant going in reverse. Now, if we assume this takes place very quickly, then equilibrium is established very quickly. And if equilibrium is established, what that means is the rate of the four reaction is equal to the rate of the reverse reaction. And so we can basically determine what the rate law is of this reaction and the reverse reaction and we set them equal.