thanks guys
The notation (A-B) also written as (A\B) is known as set difference: the set of elements of A that do not belong to B.
So $\displaystyle \left( {A\backslash B} \right) = A \cap B^c $ that is A intersection with the complement of B.
I will help with part b.
$\displaystyle \begin{array}{l}
\left( {A\backslash B} \right) \cup \left( {A\backslash C} \right) \\
\left( {A \cap B^c } \right) \cup \left( {A \cap C^c } \right) \\
A \cap \left( {B^c \cup C^c } \right) \\
A \cap \left( {B \cap C} \right)^c \\
A\backslash \left( {B \cap C} \right) \\
\end{array}$
For symbols see http://www.mathhelpforum.com/math-he...-tutorial.html