Chapter 5 - Exponents and Polynomials - 5.1 - Addition and Subtraction of Polynomials - Problem Set 5.1 - Page 195: 56

$-12x^2+8x-1$

By using the distributive property, removing the grouping symbols, and then combining like terms, the given expression, $ (3x^2+x-6)-(8x^2-9x+1)-(7x^2+2x-6) ,$ simplifies to \begin{array}{l}\require{cancel} 3x^2+x-6-8x^2+9x-1-7x^2-2x+6 \\\\= (3x^2-8x^2-7x^2)+(x+9x-2x)+(-6-1+6) \\\\= -12x^2+8x-1 .\end{array}