Select the shape. While holding down the CTRL key, grab the top center handle of the bounding box and drag downward. As you drag you'll see an outline of the mirrored-duplicate appear, then before releasing the CTRL key, right-click with the mouse to place the duplicate. Easier to do than to describe!
You can also use the Mirror tools on the main property bar or use the Mirror tools on the Tranform docker. You will need to duplicate your shape first and then move it into place.~Patti