That literally means the numbers '5' and '12' written like this: 5 + 12
represent the sum of 5 and 12, which is the same as 5 + 12 (because the sum is when all the numbers are added up).
So it's saying 5 + 12 equals to 17.
Do you get it?
x + x + 3 + x - 8
3x - 5 = 55
3x = 60, x = 20
Since we put Julie as x, we put Janet at x + 3, and the brother is x - 8 (8 years younger)
Julie is 20
Janet (x + 3) = (20 + 3) = 23
Brother(x - 8) = (20 - 8) = 12
Since we know that a number is divisible by 3 if the sum of all digits is divisible by 3.
Let us check which digits will make 371? divisible by 3.

11 is not divisible by 3.
Now let us check other digits as well.
12 is divisible by 3.

15 is also divisible by 3.

18 is divisible by 3 as well.
Therefore, 1, 4 and 7 in tenth place will make our number divisible by 3 and our numbers will be 3711, 3714 and 3717.