Answer
$3(2x-y)(2x+y)$
Work Step by Step
The product of a binomial sum and a binomial difference is: $(A+B)(A-B)=A^2-B^2$.
The square of a binomial sum is: $(A+B)^2=A^2+2AB+B^2$.
The square of a binomial difference is: $(A-B)^2=A^2-2AB+B^2$.
Hence here:
$12x^2-3y^2=3(4x^2-y^2)=3((2x)^2-y^2)=3(2x-y)(2x+y)$