Answer
$x^2+12x+36-9y^2$
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: $((x+6)+3y)((x+6)-3y)=(x+6)^2-(3y)^2=x^2+2\cdot6\cdot x+6^2-9y^2=x^2+12x+36-9y^2$
