Primary schools often have this problem of defining an operation. Sometimes these operation symbols are just written by the questioner. Never mind the original meaning of the symbol when doing the problem, just substitute the number in. In LZ problem, first take 1 as x, 2 as y, and calculate 1? 2 (of course, the result is with a), and then calculate 2 like this? 3 (the result is also a), and then calculate the value of a according to the equation. Finally, 3? 4。