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