I am trying to replace the entire p-trap assembly under my bathroom
sink. My problem is that the old drain extension tube (the part that
connects the trap to the stub-out) seems to be stuck in the stub-out.
The stub-out is cast iron and the old tube is some kind of thin metal
(yes, I am a plumbing novice.) What is the best way to remove the
tube from the stub-out without breaking the tube? Twist? Pull? Do I
need a torch or chemical to loosen it?

thanks,
Tony