need help making a solid out of surfaces

hi,

I wanted to create a solid with angeled surfaces, but I didn't know how to modify the solid so I tried to arrange surfaces that define the solid I want to create. But when I tried to create a solid out of the surfaces, Autocad keeps saying that it isn't watertight. How can I fix the problem? Is it possible to create a solid out of arranged surfaces? Or is there a better way to create the solid?

thanks