（1）（2x-6)/(x+3)÷(9-3x)/(x的平方-9)
=2（x-3）/(x+3)/3(3-x)/[(x+3)(x-3)]
=2*（x-3）^2/3
=2/3(x-3)^2 .
(2) (xy-x的平方）÷(x-y)/xy
=x(y-x)*xy/(x-y)
=-x^2y .
(3)(a-1)/(a的平方-4a+4)÷(a的平方-1）/(a的平方-4）
=[(a-1)/(a-2)^2]*[(a+2)(a-2)/(a+1)(a-1)]
=(a+2)/[(a+1)(a-2)]