Form a matrix of the coefficients of x, y &z
(1 -1 a) (x) = (1)
(2 -1 1) (y) (-1)
(a 1 -1) (z) (1)
Take multiples of the top line from the later ones to get a zero entry in the left hand entry of the other two, and then take a multiple of the (new) middle line from the...