Question 1121052
I have to assume that in your expression
A-(BnC)=(A-B)U(A-C), 'U' stands for union and 'n' stands for intersecion.
For the left the remnant is all out of B or out of C or out of both, because BnC is the elements both in B and C.
For the right A-B is all out of B and A-C is all out of C. If we take the union of them, we'll see that the union set include the elements out of B, the elements out of C, and also the ones out of both-just the same.
And it's done.