From the .bmp you provided it looks like the line may not be seen by Dolphin as intersecting with the arc. I think the easiest thing to do in this case is to use the 'Intersect' button (1st button right side of NC button) Then along the left side menu the second, third and 4th buttons from the top on the right side of the menu select how the intersected point will be clipped(Clip items,Clip first, and Split items) depending on which one you select it will clip the lines differently. Otherwise normally I would just select Edit from the top menu and then select Delete Item, and proceed to click on the line or other item you want to delete. This line will probably be deleted all the way to the corner but it is easy to redraw it from the corner to the arc with the Snap near selected(button with blue arrow pointing to a crosshatch). I hope this helps you.