The short answer questions of high school mathematics are made in China ... first correct them, then adopt them.
Because there is a, there must be 6-a, so 1 and 5 (added to 6), 2 and 4, 3 can be regarded as three elements of X, Y and Z, and then the non-empty proper subset of the set {X, Y, Z} composed of these three elements has {X} {Y} {X.Y.Z.