triangle

In my opinion, there's no point of that extra triangle, maybe just there as a red herring...

Anyways, we can use the Pythagorean theorem on 12 and 6 to get y.

We know that:

a² + b² = c²

Here is a lovely little picture that could help you wrap your mind around how it works:

So, what does this tell us? Well,  now we know this now:

12² = 6² + y²
 

This now means:

144 = 36 + y²

^Simplify:

108 = y²

Which means:

y = sqrt(108) = 6sqrt(3)

:)