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