Answer
Proof: Let A, B, and C be any sets such that x ∈ (A – B) – C
x ∈ (A – B) – C
= ( x ∈ A and x ∉ B ) and x∉ C
= x ∈ A and (x ∉ B and x∉ C)
= x ∈ A and (x ∉ B and x∉ C)
= x ∈ A and x ∉ (B U C)
= A - (B U C)
Work Step by Step
(A – B) – C = A – B – C
= A – (B U C) By set difference law