It does not change the value of a determinant to
multiply a row by a constant and add it
to another row.
Multiply the 2nd row by -x and add it to the 3rd row:
Multiply the 1st row by -x and add it to the 2nd row:
Multiply the 2nd row by -y and add it to the 3rd row:
The bottom row has all 0's, so the value of the
determinant is 0.
Edwin