This is when I get stuck. Firstly there is no cancel button. Someone told me it is the normal "backspace" button, but when I hit that it deletes the whole metal picture, not the cut off bit. Help?on the layer panel, control + click in the preview, this should select all the edges of this layer
now invert the selection (selection-->invert)
select the metal layer and hit CANC to delete everything out of the cutted off piece edges