suppose that a is an element of (X∩Y) U (Y-X).

this means one of two things:

a is in X∩Y, or

a is in Y-X

suppose a is in X∩Y. this means that a is in BOTH X and Y, so is certainly in Y.

alternatively, if a is in Y-X, this means a is in Y, but not in X. who cares if its not in X, at least it's in Y!

so in all possible cases, we see that a is in Y.

this means that (X∩Y) U (Y-X) is a subset of Y.

now suppose b is in Y.

well we have two possibilities:

b is in X

b is NOT in X (in or out, that's the way it is with sets. you cannot be "sort of" in a set).

if b is in X, then b is in X AND Y, so b is in X∩Y.

if b is not in X, then b is in Y, but not in X, so b is in Y-X.

if we put these two sets together, b is certain to be in one of them. hence b is in (X∩Y) U (Y-X).

thus Y is a subset of (X∩Y) U (Y-X).

but, if for 2 sets A,B: if A⊆B and B⊆A, then A and B have exactly the same elements, that is: A = B.

so (X∩Y) U (Y-X) = Y.

(intuitively what we are doing is splitting Y into 2 parts: the part that overlaps with X, and the part that doesn't).