Hey guys! I have a problem. the following picture shows a question on a test i had. I understand how to do part i) and part ii), but I'm not sure how to do part iii). specifically, i don't understand how they got from n(x) to N'(x)

i understand how they went from $\displaystyle 100x - 100x^4$ to $\displaystyle 100(4)x^3$ but what happened to the 2000?