x, y and z are in the AP.
So, 2y = x + z
x = 2y - z .......(1)
(x + y - z)(y + z - x) .......(2)
Substituting (1) in (2)
[(2y - z) + y - z][y + z - (2y - z)]
= (2y - z + y - z)(y + z - 2y + z)
= (3y - 2z)(2z - y)
= 6yz - 3y2 - 4z2 + 2yz
= 8yx - 3y2 - 4z2