Because the flat of the slope is the smaller the change in the y value is and the smaller our PH change is. Now, if we look at this corresponding value along our y axis, we see that it's somewhere here. And so this is equal to a PH of about 4.75. So we see that for this particular buffered system in which we use acetic acid and acetate ion for that buffered system, the greatest ability of this buffered system to resist a change in PH is when the PH is equal to 4.75. Now, what is the PKA of acetic acid? Well, if you look up the PKA value of acetic acid, it's 4.75.