The negative side is usually easy to find. I'm not there so let's just say that all laptops I've fixed in decades have the negative side shared with some hundred points such as the case shield.
Black in all units have gone to this negative connection.
I dropped my laptop on the corner and where you plug it in became out of line so i opened the comp up and the black and red wires weren't attached very well so i was going to reattach them. Time went by and i forgot what color goes on top, red or black, and what color goes on the side? Silly me. Can any one help?