The answer is 14 - x.
If she is 14 now, and the question is looking for her age x years ago, it is basically just asking you to subtract x from 14.
It also can't be x - 14, because that means Susie's age is x and the question is asking for her age 14 years ago, and it can't be 14x, because that's 14 multiplied by x.
