Because (CuA)∩(CuB) can be transformed into Cu(AUB) (this is a theorem that teachers should have said).

Obviously, it is easy to see that AUB = {1, 2,3,5,7,9}

And because (cua) ∩ b = {1, 9} means that there must be 1 and 9 in B set, but there must be no 1 and 9 in A set (obviously, 1 and 9 are in the complementary sets of A and U).

And because there must be element 2 in both a and b sets.

So a = {1, 2,9}

It can be seen from AUB = {1, 2, 3, 5, 7, 9} that:

B={2，3，5，7}

It's detailed enough. I can't help it otherwise.

I'm a senior three now, so I'll review. I hope it helps you.