For this case we have to find out how much younger sister I was three years ago.
Let the variable "x" be my age. So:
If my brother is 5 years older then:
![x + 5](https://img.qammunity.org/2019/formulas/mathematics/middle-school/o2fvodzc1h8ssf2pkzpk2u3blqo7qu8jvc.png)
If my sister is 3 years younger then:
![x-3](https://img.qammunity.org/2019/formulas/mathematics/high-school/dl27iirct56r1vu3bn11htd4m2lfc42w6u.png)
For any value of "x", that is, my age, my brother will always be 5 years older and my sister 3 years younger. Then, three years ago when I was x years old, my sister was three years younger than me.
Answer:
she was three years younger than me