Answer
$(x+6-2a)(x+6+2a)$
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: $x^2-4a^2+12x+36=\\x^2+2\cdot x\cdot6+6^2-(2a)^2\\=(x+6)^2-(2a)^2\\=(x+6-2a)(x+6+2a)$