\(x\in A\cup(B\backslash C)\Rightarrow x\in A \vee (x\in B \backslash C)\Rightarrow\)
\(x\in A \vee (x\in B\wedge x\notin C)\Rightarrow (x\in A\vee x\in B)\wedge (x\in A\vee x\notin C)\Rightarrow\)
\(x\in A\cup B\wedge \lnot(\lnot(x\in A \vee x\notin C))\Rightarrow \)
\(x\in A\cup B\wedge \lnot(x\notin A\wedge x\in C)\Rightarrow\)
\(x\in A\cup B\wedge \lnot (x\in C\backslash A)\), also
\(x\in (A\cup B)\backslash (C\backslash A)\).