12
This is 45 - 45 - 90 triangle, meaning that it has two 45 degree angles and one 90 degree angle (this is shown on the picture)
In this type of triangle, the ratio of the sides to each other is 1 - 1 - root 2.
Therefore, to get from the 1 side (leg), to the root 2 side (hypotenuse), you multiply it by root 2.
6root2 x root2 = 12
Another way is that since we know two angles are the same, we know it’s an iscosceles triangle, therefore, we know that the other leg that is not x is also 6root2. Then, from there, you could use Pythagorean’s Theorem which is a^2 + b^2 = c^2.
Hope this helps :)