Toilets, Sinks, Showers, Tubs and Disposals - Bathroom sink drain question

I have installed a new bathroom sink and am in the final stages of the plumbing hookup. I am having trouble getting the drain to not leak at the house side of the J-trap. That side of the J-trap goes into the standard plastic pipe when then connects to the metal pipe that takes the waste water out of the bathroom.

My question is this -- is it likely that the plastic piece that goes into the metal pipe is attached via threads? It kind of looks like there is a plastic pipe fitting that is permanently attached to the metal pipe, and then the piece that I want to replace screws into that permanent piece of plastic.

But I don't want to use a pipe wrench on the piece that I want to replace in case it isn't attached via threads. If I started trying to unscrew it and that is not the right way to proceed then I would likely break the plastic pipe and then I might be in a mess.

Thanks in advance.

That is a "P" trap. The part that holds the water seal is the "J" bend and the other piece is the "wall tube". The wall tube is cut to the appropriate length, deburred and a nut and washer slid onto the wall tube before sliding it into the threaded drain adapter that should be connected to the end of the pipe(which reduces the drain pipe down to the 1¼" of the wall tube). Then tighten the nut onto the threads of the drain adapter. Tighten firmly, but don't overtighten.
It sounds like you did not replace the washer. You could use either a poly washer (that looks like it was sliced off of a cone), or a rubber ring washer.