Remember that only roots with same roots you can combine.
Split 12 into 2 numbers that either one will be 3 so that you can combine the roots.
12 is the same as 4 × 3.
Root of 4 is 2 so you can take it out of the root as 2.
So the answer will be => -12(2)√3-2√3 = -22√3
Hope this helps!