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