And the reason we want to make the simplification is to basically figure out the meaning behind Km. Now, because Km appears in the denominator we can see that the units of Km are the same as that for the concentration of the substrate. And so what we're going to assume initially is we're going to set the Km value equal to the substrate concentration and we'll see why we do that in just a moment. So if we set Km equal to the substrate concentration then this denominator can be simplified from Km plus this to simply the concentration of S plus the concentration of S, where Km has been replaced with the concentration of S. So V naught is equal to v max multiplied by this ratio. Now, the denominator can be combined to basically combine these two quantities. So it's as if we have x plus x and that gives us two x.