Factor $LaTeX: \displaystyle 28 x^{3} + 20 x^{2} - 28 x - 20$ .

Factoring out the GCF $LaTeX: \displaystyle 4$ from each term gives $LaTeX: \displaystyle 4(7 x^{3} + 5 x^{2} - 7 x - 5)$ . Grouping the first two terms and factoring out their GCF, $LaTeX: \displaystyle x^{2}$ , gives $LaTeX: \displaystyle x^{2}(7 x + 5)$ . Grouping the last two terms and factoring out their GCF, $LaTeX: \displaystyle -1$ , gives $LaTeX: \displaystyle -1(7 x + 5)$ . The polynomial now has a common binomial factor of $LaTeX: \displaystyle 7 x + 5$ . This gives $LaTeX: \displaystyle 4[x^{2} \left(7 x + 5\right) -1 \cdot \left(7 x + 5\right)] = 4\left(7 x + 5\right) \left(x^{2} - 1\right)$ . The quadratic factor can be factored using the difference of squares to give $LaTeX: \displaystyle 4\left(x - 1\right) \left(x + 1\right) \left(7 x + 5\right).$