Answer
$y-6x\sqrt[]{y}+9x^2$
Work Step by Step
Using $(a+b)^2=a^2+2ab+b^2$ or the square of a binomial, then,
\begin{array}{l}
\left( \sqrt[]{y}-3x \right)^2
\\=
(\sqrt[]{y})^2+2(\sqrt[]{y})(-3x)+(-3x)^2
\\=
y-6x\sqrt[]{y}+9x^2
.\end{array}