Chapter 1 - Introduction to Algebraic Expressions - 1.6 Subtraction of Real Numbers - 1.6 Exercise Set - Page 50: 123

$8x+66$

$\bf{\text{Solution Outline:}}$ To simplify the given expression, $ 13x-(-2x)+45-(-21)-7x ,$ remove the parentheses and then combine like terms. The expression $a-(-b)$ simplifies to $a+b.$ The expression $a+(-b)$ simplifies to $a-b.$ $\bf{\text{Solution Details:}}$ Removing the parentheses and then grouping like terms result to \begin{array}{l}\require{cancel} 13x-(-2x)+45-(-21)-7x \\\\= 13x+2x+45+21-7x \\\\= (13x+2x-7x)+(45+21) \\\\= 8x+66 .\end{array}