Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$(s-6t)(s+2t)$
The 2 numbers whose product is $ac= 1(-12)=-12$ and whose sum is $b= -4$ are $\left\{ -6,2 \right\}.$ Using these two numbers to decompose the middle term of the given trinomial, $s^2-4st-12t^2 ,$ then the factored form is \begin{array}{l} s^2-6st+2st-12t^2 \\\\= (s^2-6st)+(2st-12t^2) \\\\= s(s-6t)+2t(s-6t) \\\\= s(s-6t)+2t(s-6t) \\\\= (s-6t)(s+2t) .\end{array}