First, calculate how much 12-9 is equal to 3, so the last space is 3. Then think about how much 2 plus 3 equals 3, that is, how much 3-2 equals 1, so the first one that is vacant is 1.

This is a basic calculation problem in primary schools, which tests students' reaction ability.

Primary school mathematics is to teach children a series of knowledge about number, four operations, the calculation formula of figure and length, unit conversion and so on through textbooks, which lays a good mathematical foundation for junior high school and daily life calculation.