Here we have:
 

factorising 12, we have 12=2*2*3
√12= 2√3 .............(1) 
factorising x⁶, we have x⁶ = x*x*x*x*x*x
√x⁶ = x³............(2)
factorising y¹², we have y¹² = y*y*y*y*y*y*y*y*y*y*y*y
√y¹² = y⁶...............(3)
combining (1), (2) and (3) , we have
 
 = 2x³y⁶√3