The determinate of the following 3x3 matrix
1-y, 2 , 3
2 , 4-y, 5
3 , 5 , 7-y
gives a cubic that simplifies to,
y^3 - 12*y^2 + y + 1 = 0.
Now, apparently the teacher picked random numbers for the original matrix, making the problem delve into other realms of mathematics...