Answer
$(3x+1)^{2}$
Work Step by Step
$ 9x^{2}+6x+1\qquad$...recognize a perfect square trinomial.
$(A\pm B)^{2}=A^{2}\pm 2AB+B^{2}$;
$A=3x,\ B=1$
$=(3x+1)^{2}$
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