Answer
$(9+3)+x=12+x$
Work Step by Step
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...
$=12+x$
