For this one, erase everything except for just one semi-circle, then draw a line connecting the endpoints of the half-circle. Select both those curves and then do a Construct / Extrude and that will create that type of a shape.

Any time you have something that is really straight in one direction, then that is a good candidate for extrusion.

One other thing to be aware of - your arc that is close to the origin (0,0,0) point is not quite snapped exactly on to the origin, it is a just a small amount above it. You should generally try to snap these types of objects exactly in place because it makes it easier to line up other things later on. Like in this case since it is not quite a 180 degree arc (more like 182 degrees or something), it is easy to accidentally get a some other kind of snap when you draw the line between the endpoints, like a quad snap or perp snap.

Let me know if you need more detailed instructions on any of this.

- Michael