Well, to answer this question, let's actually said these two equal to each other. So we know that the PH of our solution is equal to PKA. Now, PKA is equal to this equation negative log of ka, and ka is equal to the ratio of these two. And so, if we plug in the right side into the ka here on the right side of this equation, we basically get this. So, negative log of ka, and ka is equal to this ratio. So, we replace ka with this ratio.