Not a problem at all! I was a bit uncomfortable anyway since it was my first time doing something like this. However next time will be much better since I kind of know what to expect so we can definitely talk! However the next meat would be after your HSC wouldn't it so I am not sure if you would want to go.
Delta y is a small increment of something, so by applying summations, we can turn them into an integral sign. You don't need to really know anything beyond that. It's just the summation of disks.