If you mean that you are given two graphs, y= f(x) and y= g(x), then the "vertical separation" is the difference betwee the "y" coordinates: f(x)- g(x) or g(x)- f(x) depending on which graph is higher (it's actually, |f(x)- g(x)|). You can't put inallvalues between 0 and 3- leave it as "x".