Since we want our second shape to be the very end of the animation, we'll want to create it on the last frame; you should click on the frame 36 over from frame 12 in your timeline, frame 48.
As you can see, that frame is not currently a key frame, though our blue circle still appears there. We're going to make it a key frame, but this time we don't want our circle to stay there, so we're going to make it a blank keyframe.
