x^2 - 4
------------------------------
x + 7 | x^3 + 7x^2 - 4x - 28
x^3 + 7x^2
------------------
0 - 4x - 28
-4x - 28
-------------
0
Now we know that
x^3 + 7x^2 - 4x - 28 = (x^2 - 4)(x + 7)
x^2 - 4 can be factored since it is the difference of two squares.
x^2 - 4 = (x + 2)(x - 2)
The complete factoring is:
x^3 + 7x^2 - 4x - 28 = (x +2)(x - 2)(x + 7)