Deleting lines but end up deleting a wall and what not

SketchUp
1

I’m making an arc on this rec… but end up losing the wall when deleting the lines. please help
'5

2

What are you trying to accomplish with the ark on the slope?

3

The edges are required to create a face. If you delete an edge bounding the face, you will lose the faces that depend on that edge. why do you want to delete that edge?