#### Answer

7x(x+2)(x-2)

#### Work Step by Step

7x$^3$-28x
=7x(x$^2$-4)
Inside the parenthesis is a difference of squares. If the square root of the first term is a and the square root of the second term is b, the factorization is (a+b)(a-b).
$\sqrt {x^2}$=x and
$\sqrt {4}$=2. So the factorization is
7x(x+2)(x-2).