原式=(x-3+x+3)/(x+3)(x-3)+(x+9)/(x^2-9)
=(2x+x+9)/(x+3)(x-3)
=3(x+3)/(x+3)(x-3)
=3/(x-3)
所以x-3是3的约数
x-3=1,-1,3,-3
所以有4个