So the rate of dissociation of the enzyme substrate complex is equal to the product of the reverse reaction, k minus one multiplied by this quantity here. Now, our Es complex doesn't only dissociate going this way, there's also a probability that it will dissociate going this way. And so when it dissociates going this way, we have K two multiplied by the concentration of Es. So this basically describes the entire rate of dissociation of the enzyme substrate complex. So it can dissociate not only going this way, but also going this way. And that's why we have these two summations.