X=3y (the number of participants is three times that of non-participants)
(x- 12)=2(y+6) (according to the hypothesis, the total number of students in grade six is (x+y), according to the meaning of the question, the total number of students in this grade is (x+y-6), and the number of students who did not participate is (y+6). )
Simultaneous two equations give x = 72 and y = 24.
Therefore, the number of students who did not participate was 24, and the total number of sixth-grade students was 96.