Proof:

(® )
Let x Î A-(B È C). Then _______________ and _______________.
Since x Ï B È C, __________ and x Ï C (for if it were in either B or C then it would be in their union). Thus x Î A and x Ï B and x Ï C. Hence x Î A-B and x Î A-C, which implies that ____________________. We conclude that A-(B È C) Í (A-B) Ç (A-C).

(¬ )
Conversely, suppose that x Î __________________. Then x Î A-B and x Î A-C. But then x Î __ and x Ï __ and x Ï __. This implies that x Ï (B È C), so x Î ____________. Hence __________________ Í ________________ as desired.