For any sets A and B, use algebraic laws of sets to simplify
(a) [ A - (A-B)] - B
(b) A - [ B - (A-B)]
I don't understand your question ...
Example:
$\displaystyle X \setminus ( Y\setminus Z) = X \ ( Y \cap Z^c) = X \cap (Y \cap Z^c)^c = X \cap (Y^c \cup Z) = \\ (X \cap Y^c) \cup (X \cap Z)$
The last line is from distribution (you can check Algebra of sets - Wikipedia, the free encyclopedia for more "shortcuts")