Looks like someone put a towhook into your frame hole.
Anyway, use a wire wheel on all of those exposed adjuster threads. After cleaning it up, spray some penetrating oil on it then try to loosen the nut, while holding the cable end with a vise grips. If it looks like the nut will not move, stop or you will break off the end. Get a grinder and cut off the nut. A nut is cheaper than a new cable.