Chapter 5 - Exponents and Polynomials - 5.1 - Addition and Subtraction of Polynomials - Problem Set 5.1 - Page 195: 24

$-x^2+9x-12$

We write an expression to express this addition problem: $(-3x^2+2x-6)+(6x^2+7x+3)+(-4x^2-9)$ We use the commutative and associative properties of addition to group like terms: $(-3x^2+6x^2+(-4x^2))+(2x+7x)+(-6+(3)+(-9))$ We combine the like terms: $-x^2+9x+(-12)=-x^2+9x-12$