Answer
$x+3$
Work Step by Step
Using the rule given in the book:
If $n$ is an even positive integer, then $\sqrt[n]{a^n} = |a|$
If $n$ is an odd positive integer, then $\sqrt[n]{a^n} = a$
We get $\sqrt{x^2+6x+9} = \sqrt{(x+3)^2} \rightarrow |x+3| = x+3$