First click the "Insert" tab, select box and place it in the slide. I will assume that you know how to format the shape. Next click on the "Home" tab and in the drawing section click on the "arc" line and then place the line in your box. Make sure you format your line to be a different color than your background. Now, select your line and by manipulating the box size and the yellow diamonds on the line you can get the shape and size of the arch you need.