Chapter 1 - Introduction to Algebraic Expressions - 1.8 Exponential Notation and Order of Operations - 1.8 Exercise Set - Page 67: 100

$-3a+9$

Since the grouping symbol is preceded by a negative sign, then the sign of each term inside the grouping symbol alters once the grouping symbol is removed. Hence, the given expression, $ 2a-(5a-9) $, is equivalent to \begin{array}{l}\require{cancel} 2a-5a+9 .\end{array} By combining like terms, then this simplifies to \begin{array}{l}\require{cancel} -3a+9 .\end{array}