Amy's current age is 16 years greater than Peter's age, so Amy's current age is equal to x + 16.
6 years ago, Amy's age was equal to twice that of Peter's age at that point, so her age was 2(x - 6).
As currently her age is x + 16 then 6 years ago her age was x + 16 - 6, which is x + 10.
As 6 years ago her age was equal to 2(x - 6) and x + 10, we can see that x + 10 = 2(x - 6).
