Answer
$9x^2-6xy+y^2$
Work Step by Step
Using the square of a binomial, $(a+b)^2=a^2+2(a)(b)+b^2$, the given expression is equivalent to
\begin{align*}
&
(3x-y)^2
\\\\&
(3x)^2+2(3x)(-y)+(-y)^2
\\\\&
9x^2-6xy+y^2
.\end{align*}
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