Question

Factorise:

$28-31x-5x^2$

Answer 1

$28-31x-5x^2=28-35x+4x-5x^2\left\{\begin{array}{c}\because 28\times (-5)=-140\\ \therefore -140=-35\times 4\\ -31=-35+4\end{array}\right\}=7(4-5x)+x(4-5x)=(4-5x)(7+x)$