Chapter 1 - Some Basic Concepts of Arithmetic and Algebra - 1.5 - Use of Properties - Problem Set 1.5 - Page 34: 73

$-74$

Combining like terms, the given expression, $ 2xy+6+7xy-8 ,$ simplifies to \begin{array}{l}\require{cancel} (2xy+7xy)+(6-8) \\\\= 9xy-2 .\end{array} Substituting $ x=2 $ and $ y=-4 ,$ the resulting expression above evaluates to \begin{array}{l}\require{cancel} 9(2)(-4)-2 \\\\= -72-2 \\\\= -74 .\end{array}