Simpson,

The Rounded Rectangle has quarters of circles at the corners.

If you want to have only two straight segments, you may set the Radius to half the smaller side length.

Simpson,

You may:

1) Create a Rounded Rectangle corresponding to the desired roundings at the top,

2) Create a (sharp edged) Rectangle with the same width and half the height, bottom and centre aligned with the rounded one (Align is bundled with the Transform and Pathfinder boxes),

3) Select both and Pathfinder>Unite,

4) Delete the superfluous Anchor Points with the Delete Anchor Point Tool (bundled with the Pen Tool).

Unless you do mean that you want

the bottom left and right to be a right angel

in which case you will have to make somewhat more elaborate shapes.

Just Direct Select (the open arrow by either Shift selecting or boxing) the two bottom anchor points as indicated in the second rectangle on the image shown. Notice that the two are solid while the others are hollow. This indicates that they are active and when you hit the delete key it should look like the third but the whole path will be active. Then just use the Join command.

For my part you are welcome, Simpson.

In case you will need the method for another task where the excellent solution given by Larry is unfit,

I've created the rectangle inside it and it's al lined up but having trouble deleting the extra line.

please elaborate on the last part, and

Also, I've deleted them both and clikced pathfider although there's no unite

I presume you mean selected, and in earlier versions, Pathfinder>Unite is called Pathfinder>Add to shape area (depending on version, you may need to hold Alt/Option to get a simple path).

For my part you are welome, Simpson.