Now, if you don't know what this formula is or where it comes from, check out the link above and I'll tell you why in a second. We want to find the KB. Well, kw, something we know at some given temperature at a 25 degree celsius, kw is tens of negative 14. It's the ionization constant of water. Now, this guy equals ka, something we know times KB. So we find KB by simply dividing kw by ka.