Factorise:

The given expression is $28-31 x-5 x^{2}$.

Find two numbers that follow the conditions given below:

Sum $=-31$

Product $=28 \times-5=-140$

Clearly, the numbers are $-35$ and 4 .

$28-31 x-5 x^{2}=28+4 x-35 x-5 x^{2}$

$=4(x+7)-5 x(7+x)$

$=(x+7)(4-5 x)$