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

$-2(x-24)(x+3)$
Factoring the negative $GCF= -2$, the given expression, $-2x^2+42x+144 ,$ is equivalent to \begin{array}{l} -2(x^2-21x-72) .\end{array} The 2 numbers whose product is $ac= 1(-72)=-72$ and whose sum is $b= -21$ are $\left\{ -24,3 \right\}.$ Using these two numbers to decompose the middle term of the above trinomial, then \begin{array}{l} -2(x^2-24x+3x-72) \\\\= -2[(x^2-24x)+(3x-72)] \\\\= -2[x(x-24)+3(x-24)] \\\\= -2[(x-24)(x+3)] \\\\= -2(x-24)(x+3) .\end{array}