When you have a function, say g(x), that equals other functions added together
g(x) = f(x) + h(x) + m(x) + j(x) ...
g'(x) = f'(x) + h'(x) + m'(x) + j'(x) ...
This is from the addition rule of differentiation. There are other rules for multiplication, division, and multiplication by a constant. Here is a brief list of them: rules
Hope that is what you meant.
Wikipedia says: Sum rule in differentiation - Wikipedia, the free encyclopedia