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