Given, sides AB>AC>BC
This means, the angles opposite to the sides will also have the same relation.
=>∠ACB>∠ABC>∠BAC -- (1)
Since, xo,yo,zo are the exterior angles, they will be 180o−∠BAC;180o−∠ABC,180o−∠ACB respectively. -- (2)
Hence, from 1,2
=>180o−zo>180o−yo>180o−xo
=>zo<yo<xo