If she started with -100 points, let’s think about where that is in relation to 0. That’s just 100 points backwards from 0, right? So, in order to get to 0 again, she would have to earn 100 points ( -100 + 100 = 0 ).
So, for now, she’s earned 100, but is currently at 0 because the -100 she had and the 100 she earned cancelled out. If she finished with 800, and she’s currently at 0 (after earning 100), she would have to earn another 800 to get to her final score (0 + 800 = 800).
In total, she earned 900 points, because she earned 100 then 800, and 100+800=900.
You can also think of it this way:
-100 + x = 800
x = 800 + 100 (we added 100 to both sides)
x = 900