$(\color{red}{-8x}+\color{green}{y})^2
\,$ $=\,(\color{red}{-8x})^2 \,+ 2\times\color{red}{(-8x\,)}\times\color{green}{y} \,+ \color{green}{y}^2$ $= $ ?
$64\,x^2\, -16\,x\,y\,+ \,y^2$ $-64\,x^2\, -16\,x\,y\,+ \,y^2$ $-8\,x^2\, -16\,x\,y\,+ \,y^2$ $64x\, -16\,x\,y\,+ \,y^2$