## Introductory Algebra for College Students (7th Edition)

$(9+3)+x=12+x$
The associative property for addition of real numbers states: $(a+b)+c=a+(b+c),$ that is, changing grouping does not affect the sum. Here, $9+(3+x)=$... apply the property $=(9+3)+x=$... solve the parentheses first... $=12+x$