$(\color{red}{3y}-\color{green}{x})^2
\,$ $=\,(\color{red}{3y})^2 \,- 2\times\color{red}{3y}\times\color{green}{x} \,+ \color{green}{x}^2$ $= $ ?
$9y\, -6\,x\,y\,+ \,x^2$ $9\,y^2\, -6\,x\,y\,- \,x^2$ $9\,y^2\, -6\,x\,y\,+ \,x^2$ $3\,y^2\, -6\,x\,y\,+ \,x^2$