Focus on the top side of this new rectangle, pick the Delete Anchor Point Tool (-)and simply remove the right anchor point. Keep focusing on the top side of your new, white shape and switch to the Direct Selection Tool (A). Select the remaining anchor point and simply drag it 40px to the right. Reselect the shape made in this step and go to Object > Path > Add Anchor Points.
Make sure that the Direct Selection Tool (A) is still active, select the anchor point highlighted in the third image and simply drag it 30px up. In the end things should look like in the fourth image