Solution - Simplification or other simple results

Step  1  :

Equation at the end of step  1  :

  (3x2 -  4x) -  15

Step  2  :

Trying to factor by splitting the middle term

 2.1     Factoring  3x²-4x-15 

The first term is,  3x²  its coefficient is  3 .
The middle term is,  -4x  its coefficient is  -4 .
The last term, "the constant", is  -15 

Step-1 : Multiply the coefficient of the first term by the constant   3 • -15 = -45 

Step-2 : Find two factors of  -45  whose sum equals the coefficient of the middle term, which is   -4 .

Step-3 : Rewrite the polynomial splitting the middle term using the two factors found in step 2 above,  -9  and  5 
                     3x² - 9x + 5x - 15

Step-4 : Add up the first 2 terms, pulling out like factors :
                    3x • (x-3)
              Add up the last 2 terms, pulling out common factors :
                    5 • (x-3)
Step-5 : Add up the four terms of step 4 :
                    (3x+5)  •  (x-3)
             Which is the desired factorization

Final result :

  (x - 3) • (3x + 5)