It would be helpful to draw out a diagram but I'll explain in words.
We start at 0, ground level.
Then up a 100 metre hill, so now we are at 100m.
It plunges down 72, so we take away 72 to get 28m.
Then it takes them back up 48m, so we add 48 to get 76m.
That means that they are 76m above from where they started, ground level.
Hope I helped!
