This is easy. Destroyer has a problem just like this.
The new reaction has been flipped, so put 1 over your K value (K=2.0) --> 1/2.0
Then, since you cut the # of moles in half, you square root it (aka to the 1/2 power) --- > (1/2.0)^1/2
If the # of moles doubled, you would have squared your 1/2.0 instead of square rooting it.