Find the determinant

 Copy the first two columns again

1  0  1    1  0

0 -1 -2   0  -1

2   1  1   2   1

The determinant is  give by    [( 1 * -1 * 1) + (0) + (0)] - [ (2 * -1 * 1) + (1 *-2 * 1) + 0] =

(-1) - [ -2 + -2] =

-1 - [ -4] =

-1 + 4  =

3

I do it differently  - I just wanted to check that we get the same answer.  :)

I am expanding across the top row. :)

1(-1--2)-0+1(0--2) = 1+2=3

I have no idea how you did that Chris but I have seen some strange stuff like that in video clips before I think. :)