Verified answer

It's a technique called split the middle.

The standard form for a quadratic is Ax^2+Bx+C. To split the middle first multiply A*C. So in this case 28*-21. Look for multiples of this number you get that add up to 37. You can do this in the calculator by typing (in this case) y= -588/x and looking at what you get in the table that is whole numbers. For this one the answer is -49 and +12

so then split the -37 into those two factors:

28s^2-49s+12s-21

Then group together the terms

(28s^2-49s)(+12s-21)

Then factor each set of parentheses

7s(4s-7)+3(4s-7)

Then you can factor out 4s-7

(4s-7)(7s+3)

^^ That's the answer.

(4x-7) (7x+3)