Chapter 5 - Section 5.3 - Polynomials and Polynomial Functions - Exercise Set - Page 279: 58

$-12x^{6}-13x^{4}-23x^{2}+x+6$

We are given the expression $(8x^{4}-14x^{2}+x+6)+(-12x^{6}-21x^{4}-9x^{2})$. We can combine like terms to simplify. $-12x^{6}+(8x^{4}-21x^{4})+(-14x^{2}-9x^{2})+x+6=-12x^{6}-13x^{4}-23x^{2}+x+6$