Given,

x² – 23x + 42

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

Sum = - 23 and

Product = 42

The numbers are 21 and 2,

So,

x² – 23x + 42 = x² – 21x – 2x + 42

= x(x – 21) – 2(x – 21)

= (x – 21)(x – 2)