Factorise:

The given expression is $x^{2}-9 x+20$.

Find two numbers that follow the conditions given below :

Sum $=-9$

Product $=20$

Clearly, the numbers are $-5$ and $-4$.

$x^{2}-9 x+20=x^{2}-5 x-4 x+20$

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

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