Answer
$$(3x+1)^2 .$$
Work Step by Step
The polynomial $ 9x^2+6x+1 $ is a complete square since it is of the form $a^2+2ab+b^2=(a+b)^2$. Thus, by putting $a=3x$, $b=1$, we have
$$ 9x^2+6x+1=(3x+1)^2 .$$
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