It stands for delta of y divided by delta of x (written as) ∆y/∆x Or in other words. A change in Y divided by the change in X.

Sounds like you're doing differentials. There's probably a better place to ask this question if you need help with Calculus. I would try to explain but I struggled with this in school and i don't want to explain it wrong and leave you worse off. Maybe try Kahn Academy? If that's still a thing.

I'm not sure. My parents had a bob Marley box set and the entire thing was live albums. Like 10 CD's. It makes me think that all of his stuff was only recorded live but idk. I'll do a quick pop on Wikipedia and see if there's info.

Bob Marley.