(1), and the range of b is 2a≤x≤a+3. If b is equal to an empty set, this inequality does not hold, so 2a > a+3 is needed.

② Because B is contained in A, either B is an empty set or the range of B is covered by the range of A..

When b is an empty set, ① it has already been said.

When b is not an empty set, it must satisfy the inequality in ①, so it must be 2a≤a+3 (it can be equal here, it should be≤, and 4 is used in the figure.