When using the 30° - 60° - 90° method, the side lengths will always be x and x√3 for the legs and 2x for the hypotenuse.
Because the hypotenuse is 12, the equation is:
2x = 12
And now you find x to get the length of the shortest leg.
2x = 12
x = 6
Now that we know the length of x, you just add √3 in order to find the longest leg.
Therefore, the length of the longest leg would be 6√3.