Combine objects


wondered if anyone can help me as I'm a beginner.

I've draw two rectangles, filled, and two circles unfilled. I've arranged them and would like to combine them to produce a "U" shape that will be filled.

I can see that I need to remove the top half of the circles and join the bottom halves to the rectangles and then fill the whole object.

Would anyone direct me in how to do this 

many thanks