Let A and B be sets. Show that A\B(A\B) ⊆ B.

Answer 1

Not sure how to parse your expression.

B(A\B)

is not correct syntax, since

B is a set
(A\B) is a set, call it C

So, what does BC mean?

Similarly,

(A\B) is a set, so call it D.
Again, what is DC?

two set names juxtaposed has no meaning.

Answer 2

It's actually A \ (A \ B) ⊆ B