Dolphon question, arc over a box shape

I'm learning CAD (likely the cause of my questions), but anyway. I
draw a box (3x3), then I draw a 1/2 circle (which I want to use as a
profile for the block (i.e., end up with a block, bottom 1/2 square,
top 1/2 is arched). I cant figure out how to get Dolphin to make the
share. I tried a few things that are not giving me the results that
I'd like and I figured it would be quicker to get an answer here.

Thanks,
Ken