6(x+3)/x²-9
=6(x+3)/(x+3)(x-3)
=6/(x-3)
因为6/(x-3)是正整数
所以x-3只能为1,2,3,6
可得x的值为4,5,6,9
注:这是x的值也应为整数,不然有无数个值