12/（m²-9）-2/（m-3）
=12/(m+3)(m-3)-2(m+3)/(m+3)(m-3)
=(12-2m-6)/(m+3)(m-3)
=-2(m+3)/(m+3)(m-3)
=-2/(m-3)
（（x²-4）/（x²-4x+4）-（x-2）/（x+2））÷x/（x-2）
=[(x+2)(x-2)/(x-2)²-(x-2)/(x+2)]×(x-2)/x
=[(x+2)(x-2)/(x-2)²-(x-2)/(x+2)]×(x-2)/x
=[(x+2)/(x-2)-(x-2)/(x+2)]×(x-2)/x
=[(x+2)²-(x-2)²]/(x+2)(x-2)×(x-2)/x
=4x/(x+2)(x-2)×(x-2)/x
=4/(x+2)