Solution - Simplification or other simple results

Step  1  :

Trying to factor by splitting the middle term

 1.1     Factoring  x²-21x+108 

The first term is,  x²  its coefficient is  1 .
The middle term is,  -21x  its coefficient is  -21 .
The last term, "the constant", is  +108 

Step-1 : Multiply the coefficient of the first term by the constant   1 • 108 = 108 

Step-2 : Find two factors of  108  whose sum equals the coefficient of the middle term, which is   -21 .

Step-3 : Rewrite the polynomial splitting the middle term using the two factors found in step 2 above,  -12  and  -9 
                     x² - 12x - 9x - 108

Step-4 : Add up the first 2 terms, pulling out like factors :
                    x • (x-12)
              Add up the last 2 terms, pulling out common factors :
                    9 • (x-12)
Step-5 : Add up the four terms of step 4 :
                    (x-9)  •  (x-12)
             Which is the desired factorization

Final result :

  (x - 9) • (x - 12)