知x：y=3:2,y：z=3:5,
x:y:z = 9:6:10
令x y z分别为 9k 6k 10k
求（x+y-z）/（x+2y-3z）的值.
= (9+6-10 )k / (9+12-30) k
= 5 / (-9)
= -5/9