Two years ago, Andy was $n$ years older than Belle. Andy is twice her age now, how old was Belle 2 years ago?

1. Identify the constant difference in their ages.

The difference in age between two people remains constant over time. Since Andy was $n$ years older than Belle two years ago, Andy is still $n$ years older than Belle now.

2. Formulate an equation for their current ages and solve for Belle's current age.

Let Belle's current age be $B$.
Andy's current age is $B + n$.
We are given that Andy is twice Belle's age now:
$B + n = 2B$
$B = n$

3. Find Belle's age 2 years ago.

Belle's age 2 years ago was:
$B - 2 = n - 2$