x-y=3.①
x+2y=a-3.②
①×2+②得3x=a+3,得2x=2a/3+2
②-①得3y=a-6,得y=a/3-2
2x＞y
2a/3+2＞a/3-2
a/3＞-4
a＞-12
a的取值范围是a＞-12