How to make a rectangular box with different heights

Trying to design a built in bookshelves unit on my living room wall & my ceiling is vaulted. 7’ high to 9’ high. When I use the rectangle tool it only lets me do equal sides. Tried to use the line tool for the extra top of it, but it will not let me use the pushup tool to make it 3D. I know this is elementary beginner level stuff but I’ve already wasted 2 days trying to figure it out. Videos on You Tube are not helping. thanx!!

Drawing Basic Shapes | SketchUp Help

Pushing and Pulling Shapes into 3D | SketchUp Help

Here’s a simple version of what I think you mean.
Adding your model would allow us to be more exact.

Or if you mean vaulted with an arc.