Chapter 5 Polynomials and Polynomial Functions - 5.3 Add, Subtract, and Multiply Polynomials - 5.3 Exercises - Skill Practice - Page 349: 29

$$x^3-3x^2-25x-21$$

To multiply out three binomials, we first multiply the first two like normal, giving us two binomials total. Next, we multiply out these two binomials, multiplying each term of one binomial by each term in the other binomial. Thus, we find: $$\left(x+1\right)\left(x-7\right)\left(x+3\right) \\ \left(x^2-6x-7\right)\left(x+3\right) \\ x^3-3x^2-25x-21$$