15x^2-x-28

((3•5x²) - x) - 28 = 0

Step  2  :

Trying to factor by splitting the middle term

  Factoring  15x²-x-28

The first term is,  15x2  its coefficient is  15 .
The middle term is,  -x  its coefficient is  -1 .
The last term, "the constant", is  -28

Step-1 : Multiply the coefficient of the first term by the constant  15 • -28 = -420

Step-2 : Find two factors of  -420  whose sum equals the coefficient of the middle term, which is   -1 .
Step-3 : Rewrite the polynomial splitting the middle term using the two factors found in step 2 above,  -21  and  20
                     15x2 - 21x + 20x - 28

Step-4 : Add up the first 2 terms, pulling out like factors :
                    3x • (5x-7)
              Add up the last 2 terms, pulling out common factors :
                    4 • (5x-7)
Step-5 : Add up the four terms of step 4 :
                    (3x+4)  •  (5x-7)
             Which is the desired factorization

Equation at the end of step  2  :

(5x - 7) • (3x + 4) = 0