the factory---iam going to is the place ---my mother used to

that ...where
这是两个定语从句,第一个修饰 the factory,同时引导词在从句中作宾语,所以用 that,当然 which 也可以,第二个修饰 the place,同时引导词在从句中作地点状语,所以引导词用疑问副词 where.