$(\color{red}{-2x}-\color{green}{y})^2
\,$ $=\,(\color{red}{-2x})^2 \,- 2\times\color{red}{(-2x\,)}\times\color{green}{y} \,+ \color{green}{y}^2$ $= $ ?
$4x\, +4\,x\,y\,+ \,y^2$ $4\,x^2\, +4\,x\,y\,- \,y^2$ $-2\,x^2\, +4\,x\,y\,+ \,y^2$ $4\,x^2\, +4\,x\,y\,+ \,y^2$