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