Juan,

If you add the numbers from left to right the way we read it then you have 2 + 4 = 6. Next you have 6 + 6 to get 12. What the question is suggesting is that if you look at the three numbers and realize that 4 + 6 = 10 then the second step is easier, 2 + 10 = 12.

What I did was group the 4 and 6 together first. Using mathematical notation

2 + 4 + 6 = 2 + (4 + 6) = 2 + 10 = 12.

I hope this helps,
Penny