Chapter 11 - Rational Expressions and Functions - 11-3 Dividing Polynomials - Practice and Problem-Solving Exercises - Page 682: 43

The answer is $2r^4+r^2-7$

$(21-5r^4-10r^2+2r^6) \div (r^2-3)$ $=(2r^6-5r^4-10r^2+21) \div (r^2-3)$ $-16c^2-20c-25$ _________________________ $r^2-3) 2r^6-5r^4-10r^2+21$ $-2r^6+6r^4$ _________________________ $r^4-10r^2$ $-r^4+3r^2$ _________________________ $-7r^2 +21$ $-7r^2 +21$ _________________________ $0$ The answer is $2r^4+r^2-7$.