8. $(x + 1)(x - 1)(x + 3)$

Explanation:

Step1: Multiply first two binomials

Use difference - of - squares formula $(a + b)(a - b)=a^{2}-b^{2}$. Here $a = x$ and $b = 1$, so $(x + 1)(x - 1)=x^{2}-1$.

Step2: Multiply the result by the third binomial

$(x^{2}-1)(x + 3)=x^{2}(x + 3)-1(x + 3)$.

Step3: Expand the products

$x^{2}(x + 3)-1(x + 3)=x^{3}+3x^{2}-x - 3$.

Answer:

$x^{3}+3x^{2}-x - 3$