x² – 9x + 20

Now, first we have to find out the numbers whose-

Sum = - 9 and

Product = 20

The numbers are 5 and 4,

So,

x² – 9x + 20 = x² – 5x – 4x + 20

= x(x – 5) – 4(x – 5)

= (x – 5)(x – 4)