If these are vector elements you're trying to merge I believe you need create a rectangle with the top portion deleted from the area where the arc is. Then you'll need to select both elements and place the arc nodes on the rectangle nodes until you see the "crosshairs" appear. That indicates that the two nodes are joined. Do this for both ends of the arc and it will then form a single vector that you can use for a carve region or other element.
Hope that helps.
Tracy
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