Answer
$$3(x+2)(x-6)$$
Work Step by Step
We take out the constant $3$ from the given polynomial:
$$3x^2 - 12x - 36=3(x^2 - 4x - 12)$$
To factor the remaining polynomial, we have to find integers whose product is $-4$ and whose sum is $-6$. That is, we have $2$ and $-6$ where $2-6=-4$ and $2\times (-6)=-12$. Thus
$$ 3(x^2 - 4x - 12)=3(x+2)(x-6) .$$
